Although vast numbers of educational innovations have been introduced into the nation's schools over the past 20 years, few have succeeded in effecting affective, behavioral, and cognitive student gains. Many recent studies have focused, therefore, on school change processes and the formulation of strategies for successful innovation implementation. These studies, however, have not formulated the means for determining if, or how, individual teachers have integrated innovation into their regular classroom practice. This paper offers a definition of institutionalization that addresses this problem, first by describing five subprocesses (assessment of present practices, response adoption, initiation, implementation, and institutionalization); then by offering a set of three vectors for describing innovation use (Stages of Concern, Levels of Use, and Innovation Configuration); and finally by explaining how these vectors can be used to determine if an innovation has become institutionalized. The paper concludes with implications for policy determination, intervention, and evaluation. A list of 27 references is appended. (IW)