The existence of noticeable 'unified' parties is central to the theory and practice of democracy in general, and to parliamentary democracy in particular. However, legislative studies scholars have good reason to cease treating parties as monolithic, unitary actors, for they evidently are not. The first step in this direction is to ask why one of the distinguishing features of modern political parties is their legislative unity. Do parties enter parliament as unified actors, or are they moulded into this model by the legislature? The answer depends on whether one is looking at cohesion or at discipline. The goal of this collection of articles is to present a conceptual delineation between these two key concepts. This book was previously published as a special issue of the Journal of Legislative Studies .