This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. It reviews main approaches required to understand the different facets of the Trotskyist organization, drawing first on the political science literature and the analysis of political parties as organizations seeking votes, office and policy implementation. The book describes the development of Trotsky's thought and political activity and its emergence from within Russian Bolshevism, avoiding the intense hostility of critics. It describes the evolution of British Trotskyism from 1950 to 2017. The book explores in detail the shifting loci of recruitment since the 1950s, particularly in light of the sharp decline in trade union membership since 1979. It considers elections and electoral performance from 1974, the first time Trotskyist candidates participated in a British general election, up to 2017. The book straddles the party and social movement faces of Trotskyist groups by analysing four types of resources they can acquire and mobilize in campaigns.