This paper defines and illustrates content-based language teaching in the ESL context. Some good programs are cited as examples, and the reader is given an introduction to the theoretical and practical motivations of content-based ESL, along with some notion of who the principle innovators are. Three content-based models (theme-based, adjunct, and sheltered) are presented in some detail. Since content-based ESL fits so well with current principles of communicative second language teaching, the author argues that content-based ESL, tempered with judicious use of humanistically motivated experiential activities, will be the major approach to formal ESL instruction at all levels in the near future.