This paper describes a model of classroom processes that can be used in research on the nature of the relationship between school practices and student achievement. It is stated that the model specifies that criterion performance is a function of initial abilities and of the following four constructs: (1) opportunity, (2) motivators, (3) structure, and (4) instructional events. The paper describes each construct as it pertains to the classroom and deals with evaluating the model and testing the assumptions. The model is applied to the Ashton-Warner approach, the Montessori system, and the individualized programs developed by the Learning Research and Development Center. The paper then describes the variables that make up each of the four constructs and two additional constructs (initial performance, and outcomes). Lastly, the paper deals with the construction of measures of the previously identified variables. The problems involved in moving from variables to measures are discussed, methods that have been found to be useful in describing classroom processes are summarized, and an example of a classroom data-collection instrument is described. (RC)