This paper argues that research on school effects has come to regard instruction as the core of the schooling process. The paper adopts an organizational view of schools which suggests that instruction is the school's "core technology," occurring when teachers apply available resources such as time and curricular materials to the exigencies of classroom life. Two aspects of instruction, the use of time and the coverage of curricular content, are discussed in detail. But the paper also acknowledges that instruction occurs in a social context, which must be specified if one wishes to analyze schooling rather than pedagogy. Finally, an empirical example is presented to illustrate the formulation and to suggest further ways of measuring instruction. (Author)