"Ambush" is the premiere episode of the fourth season of the American medical drama ER. The 70th episode overall, it was written by executive producer Carol Flint and directed by Thomas Schlamme and it was first broadcast on NBC on September 25, 1997, as a live episode, filmed twice for the East and West Coast. The episode was also pre-filmed in the event a technical issue happened.

The fourth season of the American fictional drama television series ER first aired on September 25, 1997, and concluded on May 14, 1998. The fourth season consists of 22 episodes.