diff --git a/doc/release-notes/rl-2009.adoc b/doc/release-notes/rl-2009.adoc
index 0562e046d..90d546600 100644
--- a/doc/release-notes/rl-2009.adoc
+++ b/doc/release-notes/rl-2009.adoc
@@ -6,10 +6,13 @@ section is therefore not final.
 
 [[sec-release-20.09-highlights]]
 === Highlights
+:sd-switch-url: https://gitlab.com/rycee/sd-switch
 
 This release has the following notable changes:
 
-* Nothing has happened.
+* The systemd activation is now handled by {sd-switch-url}[sd-switch], a program that stops, starts, reloads, etc. systemd units as necessary to match the new Home Manager configuration.
++
+Since sd-switch is relatively lightweight it is always used and the option `systemd.user.startServices` is therefore considered obsolete and can be removed from your configuration.
 
 [[sec-release-20.09-state-version-changes]]
 === State Version Changes
diff --git a/modules/misc/news.nix b/modules/misc/news.nix
index 62f57a44c..fae018691 100644
--- a/modules/misc/news.nix
+++ b/modules/misc/news.nix
@@ -1619,6 +1619,21 @@ in
           A new module is available: 'services.dropbox'.
         '';
       }
+
+      {
+        time = "2020-08-03T22:34:42+00:00";
+        condition = hostPlatform.isLinux && (with config.systemd.user;
+          services != {} || sockets != {} || targets != {} || timers != {});
+        message = ''
+          The systemd activation is now handled by 'sd-switch', a program that
+          stops, starts, reloads, etc. systemd units as necessary to match the
+          new Home Manager configuration.
+
+          Since sd-switch is relatively lightweight it is always used and the
+          option 'systemd.user.startServices' is therefore considered obsolete
+          and can be removed from your configuration.
+        '';
+      }
     ];
   };
 }
diff --git a/modules/systemd-activate.rb b/modules/systemd-activate.rb
deleted file mode 100644
index 31d06d8fc..000000000
--- a/modules/systemd-activate.rb
+++ /dev/null
@@ -1,216 +0,0 @@
-require 'set'
-require 'open3'
-
-@dry_run = ENV['DRY_RUN']
-@verbose = ENV['VERBOSE']
-
-UnitsDir = 'home-files/.config/systemd/user'
-
-# 1. Stop all services from the old generation that are not present in the new generation.
-# 2. Ensure all services from the new generation that are wanted by active targets are running:
-#    - Start services that are not already running.
-#    - Restart services whose unit config files have changed between generations.
-# 3. If any services were (re)started, wait 'start_timeout_ms' and report services
-#    that failed to start. This helps debugging quickly failing services.
-#
-# Whenever service failures are detected, show the output of
-# 'systemd --user status' for the affected services.
-#
-def setup_services(old_gen_path, new_gen_path, start_timeout_ms_string)
-  start_timeout_ms = start_timeout_ms_string.to_i
-
-  old_units_path = File.join(old_gen_path, UnitsDir) unless old_gen_path.empty?
-  new_units_path = File.join(new_gen_path, UnitsDir)
-
-  old_services = get_services(old_units_path)
-  new_services = get_services(new_units_path)
-
-  exit if old_services.empty? && new_services.empty?
-
-  all_services = get_active_targets_units(new_units_path)
-  maybe_changed = all_services & old_services
-  changed_services = get_changed_services(old_units_path, new_units_path, maybe_changed)
-  unchanged_oneshots = get_oneshot_services(maybe_changed - changed_services)
-
-  # These services should be running when this script is finished
-  services_to_run = all_services - unchanged_oneshots
-
-  # Only stop active services, otherwise we might get a 'service not loaded' error
-  # for inactive services that were removed in the current generation.
-  to_stop = get_active_units(old_services - new_services)
-  to_restart = changed_services
-  to_start = get_inactive_units(services_to_run - to_restart)
-
-  raise "daemon-reload failed" unless run_cmd('systemctl', '--user', 'daemon-reload')
-
-  # Exclude units that shouldn't be (re)started or stopped
-  no_manual_start, no_manual_stop, no_restart = get_restricted_units(to_stop + to_restart + to_start)
-  notify_skipped_units(to_restart & no_restart)
-  to_stop -= no_manual_stop
-  to_restart -= no_manual_stop + no_manual_start + no_restart
-  to_start -= no_manual_start
-
-  if to_stop.empty? && to_start.empty? && to_restart.empty?
-    print_service_msg("All services are already running", services_to_run)
-  else
-    puts "Setting up services" if @verbose
-    systemctl_action('stop', to_stop)
-    systemctl_action('start', to_start)
-    systemctl_action('restart', to_restart)
-    started_services = to_start + to_restart
-    if start_timeout_ms > 0 && !started_services.empty? && !@dry_run
-      failed = wait_and_get_failed_services(started_services, start_timeout_ms)
-      if failed.empty?
-        print_service_msg("All services are running", services_to_run)
-      else
-        puts
-        puts "Error. These services failed to start:", failed
-        show_failed_services_status(failed)
-        exit 1
-      end
-    end
-  end
-end
-
-def get_services(dir)
-  services = get_service_files(dir) if dir && Dir.exists?(dir)
-  Set.new(services)
-end
-
-def get_service_files(dir)
-  Dir.chdir(dir) { Dir['*[^@].{service,socket,timer}'] }
-end
-
-def get_changed_services(dir_a, dir_b, services)
-  services.select do |service|
-    a = File.join(dir_a, service)
-    b = File.join(dir_b, service)
-    (File.size(a) != File.size(b)) || (File.read(a) != File.read(b))
-  end
-end
-
-TargetDirRegexp = /^(.*\.target)\.wants$/
-
-# @return all units wanted by active targets
-def get_active_targets_units(units_dir)
-  return Set.new unless Dir.exists?(units_dir)
-  targets = Dir.entries(units_dir).map { |entry| entry[TargetDirRegexp, 1] }.compact
-  active_targets = get_active_units(targets)
-  active_units = active_targets.map do |target|
-    get_service_files(File.join(units_dir, "#{target}.wants"))
-  end.flatten
-  Set.new(active_units)
-end
-
-# @return true on success
-def run_cmd(*cmd)
-  print_cmd cmd
-  @dry_run || system(*cmd)
-end
-
-def systemctl_action(cmd, services)
-  return if services.empty?
-
-  verb = (cmd == 'stop') ? 'Stopping' : "#{cmd.capitalize}ing"
-  puts "#{verb}: #{services.join(' ')}"
-
-  cmd = ['systemctl', '--user', cmd, *services]
-  if @dry_run
-    puts cmd.join(' ')
-    return
-  end
-
-  output, status = Open3.capture2e(*cmd)
-  print output
-  # Show status for failed services
-  unless status.success?
-    # Due to a bug in systemd, the '--user' argument is not always provided
-    output.scan(/systemctl (?:--user )?(status .*?)['"]/).flatten.each do |status_cmd|
-      puts
-      run_cmd("systemctl --user #{status_cmd}")
-    end
-    exit 1
-  end
-end
-
-def systemctl(*cmd)
-  output, _ = Open3.capture2('systemctl', '--user', *cmd)
-  output
-end
-
-def print_cmd(cmd)
-  puts [*cmd].join(' ') if @verbose || @dry_run
-end
-
-def get_active_units(units)
-  filter_units(units) { |state| state == 'active' }
-end
-
-def get_inactive_units(units)
-  filter_units(units) { |state| state != 'active' }
-end
-
-def get_failed_units(units)
-  filter_units(units) { |state| state == 'failed' }
-end
-
-def filter_units(units)
-  return [] if units.empty?
-  states = systemctl('is-active', *units).split
-  units.select.with_index { |_, i| yield states[i] }
-end
-
-def get_oneshot_services(units)
-  return [] if units.empty?
-  types = systemctl('show', '-p', 'Type', *units).split
-  units.select.with_index do |_, i|
-    types[i] == 'Type=oneshot'
-  end
-end
-
-def get_restricted_units(units)
-  infos = systemctl('show', '-p', 'RefuseManualStart', '-p', 'RefuseManualStop', *units)
-          .split("\n\n")
-  no_manual_start = []
-  no_manual_stop = []
-  infos.zip(units).each do |info, unit|
-    no_start, no_stop = info.split("\n")
-    no_manual_start << unit if no_start.end_with?('yes')
-    no_manual_stop << unit if no_stop.end_with?('yes')
-  end
-  # Get units that should not be restarted even if a change has been detected.
-  no_restart_regexp = /^\s*X-RestartIfChanged\s*=\s*false\b/
-  no_restart = units.select { |unit| systemctl('cat', unit) =~ no_restart_regexp }
-  [no_manual_start, no_manual_stop, no_restart]
-end
-
-def wait_and_get_failed_services(services, start_timeout_ms)
-  puts "Waiting #{start_timeout_ms} ms for services to fail"
-  # Force the previous message to always be visible before sleeping
-  STDOUT.flush
-  sleep(start_timeout_ms / 1000.0)
-  get_failed_units(services)
-end
-
-def show_failed_services_status(services)
-  puts
-  services.each do |service|
-    run_cmd('systemctl', '--user', 'status', service)
-    puts
-  end
-end
-
-def print_service_msg(msg, services)
-  return if services.empty?
-  if @verbose
-    puts "#{msg}:", services.to_a
-  else
-    puts msg
-  end
-end
-
-def notify_skipped_units(no_restart)
-  puts "Not restarting: #{no_restart.join(' ')}" unless no_restart.empty?
-end
-
-setup_services(*ARGV)
diff --git a/modules/systemd-activate.sh b/modules/systemd-activate.sh
deleted file mode 100644
index 1c464693c..000000000
--- a/modules/systemd-activate.sh
+++ /dev/null
@@ -1,114 +0,0 @@
-#!/usr/bin/env bash
-
-function isStartable() {
-  local service="$1"
-  [[ $(systemctl --user show -p RefuseManualStart "$service") == *=no ]]
-}
-
-function isStoppable() {
-  if [[ -v oldGenPath ]] ; then
-    local service="$1"
-    [[ $(systemctl --user show -p RefuseManualStop "$service") == *=no ]]
-  fi
-}
-
-function systemdPostReload() {
-  local workDir
-  workDir="$(mktemp -d)"
-
-  if [[ -v oldGenPath ]] ; then
-    local oldUserServicePath="$oldGenPath/home-files/.config/systemd/user"
-  fi
-
-  local newUserServicePath="$newGenPath/home-files/.config/systemd/user"
-  local oldServiceFiles="$workDir/old-files"
-  local newServiceFiles="$workDir/new-files"
-  local servicesDiffFile="$workDir/diff-files"
-
-  if [[ ! (-v oldUserServicePath && -d "$oldUserServicePath") \
-      && ! -d "$newUserServicePath" ]]; then
-    return
-  fi
-
-  if [[ ! (-v oldUserServicePath && -d "$oldUserServicePath") ]]; then
-    touch "$oldServiceFiles"
-  else
-    find "$oldUserServicePath" \
-      -maxdepth 1 -name '*.service' -exec basename '{}' ';' \
-      | sort \
-      > "$oldServiceFiles"
-  fi
-
-  if [[ ! -d "$newUserServicePath" ]]; then
-    touch "$newServiceFiles"
-  else
-    find "$newUserServicePath" \
-      -maxdepth 1 -name '*.service' -exec basename '{}' ';' \
-      | sort \
-      > "$newServiceFiles"
-  fi
-
-  diff \
-    --new-line-format='+%L' \
-    --old-line-format='-%L' \
-    --unchanged-line-format=' %L' \
-    "$oldServiceFiles" "$newServiceFiles" \
-    > "$servicesDiffFile" || true
-
-  local -a maybeRestart=( $(grep '^ ' "$servicesDiffFile" | cut -c2-) )
-  local -a maybeStop=( $(grep '^-' "$servicesDiffFile" | cut -c2-) )
-  local -a maybeStart=( $(grep '^+' "$servicesDiffFile" | cut -c2-) )
-  local -a toRestart=( )
-  local -a toStop=( )
-  local -a toStart=( )
-
-  for f in "${maybeRestart[@]}" ; do
-    if isStoppable "$f" \
-        && isStartable "$f" \
-        && systemctl --quiet --user is-active "$f" \
-        && ! cmp --quiet \
-            "$oldUserServicePath/$f" \
-            "$newUserServicePath/$f" ; then
-      toRestart+=("$f")
-    fi
-  done
-
-  for f in "${maybeStop[@]}" ; do
-    if isStoppable "$f" ; then
-      toStop+=("$f")
-    fi
-  done
-
-  for f in "${maybeStart[@]}" ; do
-    if isStartable "$f" ; then
-      toStart+=("$f")
-    fi
-  done
-
-  rm -r "$workDir"
-
-  local sugg=""
-
-  if [[ -n "${toRestart[@]}" ]] ; then
-    sugg="${sugg}systemctl --user restart ${toRestart[@]}\n"
-  fi
-
-  if [[ -n "${toStop[@]}" ]] ; then
-    sugg="${sugg}systemctl --user stop ${toStop[@]}\n"
-  fi
-
-  if [[ -n "${toStart[@]}" ]] ; then
-    sugg="${sugg}systemctl --user start ${toStart[@]}\n"
-  fi
-
-  if [[ -n "$sugg" ]] ; then
-    echo "Suggested commands:"
-    echo -n -e "$sugg"
-  fi
-}
-
-oldGenPath="$1"
-newGenPath="$2"
-
-$DRY_RUN_CMD systemctl --user daemon-reload
-systemdPostReload
diff --git a/modules/systemd.nix b/modules/systemd.nix
index dcb1a2957..5acfabc74 100644
--- a/modules/systemd.nix
+++ b/modules/systemd.nix
@@ -54,8 +54,6 @@ let
   buildServices = style: serviceCfgs:
     concatLists (mapAttrsToList (buildService style) serviceCfgs);
 
-  servicesStartTimeoutMs = builtins.toString cfg.servicesStartTimeoutMs;
-
   unitType = unitKind: with types;
     let
       primitive = either bool (either int str);
@@ -152,9 +150,11 @@ in
         example = unitExample "Path";
       };
 
+      # Keep for a while for backwards compatibility.
       startServices = mkOption {
         default = false;
         type = types.bool;
+        visible = false;
         description = ''
           Start all services that are wanted by active targets.
           Additionally, stop obsolete services from the previous
@@ -164,10 +164,10 @@ in
 
       servicesStartTimeoutMs = mkOption {
         default = 0;
-        type = types.int;
+        type = types.ints.unsigned;
         description = ''
-          How long to wait for started services to fail until their
-          start is considered successful.
+          How long to wait for started services to fail until their start is
+          considered successful. The value 0 indicates no timeout.
         '';
       };
 
@@ -203,6 +203,10 @@ in
               "Must use Linux for modules that require systemd: " + names;
         }
       ];
+
+      warnings = mkIf cfg.startServices [
+        "The option 'systemd.user.startServices' is obsolete and can be removed."
+      ];
     }
 
     # If we run under a Linux system we assume that systemd is
@@ -230,13 +234,17 @@ in
       # set it ourselves in that case.
       home.activation.reloadSystemD = hm.dag.entryAfter ["linkGeneration"] (
         let
-          autoReloadCmd = ''
-            ${pkgs.ruby}/bin/ruby ${./systemd-activate.rb} \
-              "''${oldGenPath=}" "$newGenPath" "${servicesStartTimeoutMs}"
-          '';
+          timeoutArg =
+            if cfg.servicesStartTimeoutMs != 0 then
+              "--timeout " + toString cfg.servicesStartTimeoutMs
+            else
+              "";
 
-          legacyReloadCmd = ''
-            bash ${./systemd-activate.sh} "''${oldGenPath=}" "$newGenPath"
+          sdSwitchCmd = ''
+            ${pkgs.sd-switch}/bin/sd-switch \
+              ''${DRY_RUN:+--dry-run} $VERBOSE_ARG ${timeoutArg} \
+              ''${oldGenPath:+--old-units $oldGenPath/home-files/.config/systemd/user} \
+              --new-units $newGenPath/home-files/.config/systemd/user
           '';
 
           ensureRuntimeDir = "XDG_RUNTIME_DIR=\${XDG_RUNTIME_DIR:-/run/user/$(id -u)}";
@@ -254,8 +262,7 @@ in
               fi
 
               ${ensureRuntimeDir} \
-              PATH=${dirOf cfg.systemctlPath}:$PATH \
-                ${if cfg.startServices then autoReloadCmd else legacyReloadCmd}
+                ${sdSwitchCmd}
             else
               echo "User systemd daemon not running. Skipping reload."
             fi