The basic properties of q-vertex operators are formulated in the context of the Andrews-Baxter-Forrester (ABF) series, as an example of face interaction models, the q-difference equations satisfied by their correlation functions are derived, and their connection with representation theory established. The q-difference equations of the Kashiwara-Miwa (KM) series are discussed as an example of edge interaction models. Next, the Ising model, the simplest special case of both ABF and KM series, is studied in more detail using the Jordan-Wigner fermions. In particular, all matrix elements of vertex operators are calculated. © 1994 American Institute of Physics.