This entry in Contradictions’ ongoing “Conceptual Dictionary” examines the conceptual history of the terms “dissent” and “dissidence” and their equivalents, in English, French, German, and Czech. It demonstrates that each language has its own terminology with unique nuances of semantics and connotations that require attention when studying expressions of “left dissent” in different linguistic contexts. The second part of the text discusses how scholars and practitioners of dissidence/dissent in the Soviet Union and Eastern Europe have defined and understood these terms and their relationship to the semantically close concepts of “resistance” or “opposition.” The text then gives a brief survey of research on left dissent in the West and under state socialism, before concluding with some reflections on possible new approaches to the topic.