This chapter focuses on the impact of party size on party organizations by using a cross-national sample of Western industrial democracies. It highlights some problems - both conceptual and empirical - that have limited studies of party size in the past. The impact of organizational size on other organizational attributes is well established within the studies of complex organizations. The empirical impediments of studies in party size stem largely from the problems associated with the conceptual vagueness of the phenomenon. Numerous studies have analyzed the decline of party membership, while none have studied the impact of this decline on party organizations themselves. "The issue of party decline is, therefore, not only a matter of establishing time-series data for appropriate indicators but also of relating such indicators to organizational changes in the parties and to contextual factors that may modify their impact". The chapter also presents an overview of the key concepts discussed in this book.