Institutionalization as a 'process theory' moves beyond the specification of a dichotomous variable - either an institution exists or it does not - to a continuous variable - whereby an institution can be more or less institutionalized. But if institutionalization is to be conceived in this way, then the specification of independent variables and the measurement of the degree of institutionalization become crucial to the understanding of the process. The essence of the argument presented in this article is that in the study of legislatures both the specification and measurement of institutionalization have been deficient. The comparative potential of the concept of legislative institutionalization has been limited by a failure to use common criteria and measures of institutionalization. As a general theory, institutionalization is of value in explaining how institutions become organized and how they become differentiated from other political organizations, but equally other organization theories might more usefully be deployed to explain institutional change in legislatures thereafter.