Entryism (also referred to as entrism, occasionally as enterism) is a political strategy in which an organisation or state encourages its members or supporters to join another, usually larger organisation in an attempt to expand influence and expand their ideas and program. In situations where the organization being "entered" is hostile to entrism, the entrists may engage in a degree of subterfuge to hide the fact that they are an organisation in their own right.
In entryism sui generis ("of a special type"), Trotskyists, for example, do not openly argue for the building of a Trotskyist party. "Deep entryism" refers to the long duration.
The tactic is closely identified with Michel Pablo and Gerry Healy, who were leaders of the Fourth International in the late 1940s and 1950s. The "deep entry" tactic was developed as a way for Trotskyists to respond to the Cold War. In countries where there were mass social democratic or communist parties, it was as difficult to be accepted into these parties as Trotskyist currents as to build separate Trotskyist parties. Therefore, Trotskyists were advised to join the mass party.
In Europe, this was the approach used, for example, by The Club in the Labour Party, and by Fourth Internationalists inside the Communist Parties. In France, Trotskyist organizations, most notably the Parti des Travailleurs and its predecessors, have successfully entered trade unions and mainstream left-wing parties.