ESP teaching often takes, as a point of departure, the analysis and description of language systems. Hopper (1987) de nes language systems as a set of abstract structures present for all speakers and hearers that is a prerequisite for the use of language. This chapter examines three language systems evident in ESP teaching and research. The rst two of these, grammatical structures and core vocabulary, are examined together rst, and the third, patterns of text organization, is examined in the last section.