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final Handler handler = new Handler();
handler.postDelayed(new Runnable() {
  @Override
  public void run() {
    //Do something after 100ms
  }
}, 100);

A Handler must always be linked to a Looper, which will actually process the Runnable you post(). The UI thread already comes with a Looper, so you can just make a new Handler() on the UI thread and post() Runnables directly to it. These Runnables execute on the UI thread. To have Runnables execute on another thread, you need to make a new thread, then Looper.prepare(), make a new Handler() and then Looper.loop(). Any Runnables posted to this new Handler will execute on this new thread. If you don't do all this, the post() will throw an exception.