This book examines entryism in the context of the revolutionary socialist left in Britain, from the inception of the Communist Party of Great Britain in 1920 to the departure of Militant from the Labour Party in 1992. Entryism is a tactic of penetration of a political party by another, aimed at accomplishing objectives, the nature of which can change depending on the type of entry. This work shows to what extent there is not one type of entryism but several. The adopted methodology is chronological, with introductory chapters that study the context and the previous partial-only attempts to define entryism. The first part of the volume is dedicated to the relationship between the Communist Party of Great Britain and the Labour Party up until the middle of the 20th century. The following two parts are dedicated to British Trotskyists before and after the Second World War. In total, 17 organisations that have practiced entryism are examined. Through their objectives, practices, and results, this work intends to formulate an exhaustive typology of the tactic, which fills a definitional gap in political science and covers an aspect of Labour’s History that has only been partially covered. This volume will be of use to students and scholars interested in the history of the Labour Party and the Far Left in the United Kingdom.