When we began this review, we intended to write a chapter entitled, Phase transitions in two-dimensional systems. It soon became evident, however, that it was impossible to cover so broad a topic within the imposed size limitation. Even with the restriction to monolayers, it is difficult to provide a complete overview of this lively and growing field. Fortunately, recent reviews by M6hwald (1) and McConnell (2) dealt with Langmuir monolayers. Because they did not focus on phase transitions, our chapter complements, rather than duplicates, this work. New experimental tech niques that have been applied to the study of monolayers have also been described recently (3); two Annual Review of Materials Science articles are also relevant (4, 5). And, Wortis (6) has evaluated the state of the art (as of 1990) in the study of surface films of small molecules in a brief and entertaining review. We have divided this chapter into four main sections. The first deals with structure and is primarily a review of experiments. Recent Monte Carlo (MC) and molecular dynamics (MD) simulations are discussed next. The kinetics of monolayer phase transitions are then described, and we