Over the span from its initial airing with a two-hour premiere in September 1994 until its final broadcast in 2009, ER was an unstoppable ratings powerhouse. It often led the annual rankings during its impressive 331-episode stint and kickstarted numerous careers, including George Clooney among others. Besides well-known actors such as Clooney, Anthony Edwards, Maura Tierney, and Eriq LaSalle on the main cast, the show also welcomed a multitude of guest stars and rising talents throughout its run. Here’s a glimpse at some of the most prominent ones: