Parsimony occupies a foundational role in the construction of scientific theories, functioning as a heuristic criterion across in all three stages of theory-building—justification, selection, and generation. It asserts that, all else being equal, the theory or hypothesis with the fewest assumptions should be preferred. Parsimony plays a fundamental role in the philosophy, logic, and science (including psychology), serving as a methodological guide advocating for simplicity and cognitive economy in theoretical explanation and inference. This entry explores the philosophical foundations and evolution of the principle, its applications in contemporary scientific practice, and its relevance in psychological models of cognition. It also addresses key debates surrounding oversimplification, ambiguity, and cognitive bias, arguing that parsimony is not merely a heuristic for scientific theory choice but a profound epistemic principle shaping our understanding of rationality and theory-building across disciplines.