  This entry outlines the role of the law of non-contradiction (LNC) as the foremost among the first (indemonstrable) principles of Aristotelian philosophy and its heirs, and depicts the relation between LNC and LEM (the law of excluded middle) in establishing the nature of contradictory and contrary opposition. §1 presents the classical treatment of LNC as an axiom in Aristotle's “First Philosophy” and reviews the status of contradictory and contrary opposition as schematized on the Square of Opposition. §2 explores in further detail the possible characterizations of LNC and LEM, including the relevance of future contingent statements in which LEM (but not LNC) is sometimes held to fail. §3 addresses the mismatch between the logical status of contradictory negation as a propositional operator and the diverse realizations of contradictory negation within natural language. §4 deals with several challenges to LNC within Western philosophy, including the paradoxes, and the relation between systems with truth-value gaps (violating LEM) and those with truth-value gluts (violating LNC).  In §5, the tetralemma of Buddhist logic is discussed within the context of gaps and gluts; it is suggested that apparent violations of LNC in this tradition (and others) can be attributed to either differing viewpoints of evaluation, as foreseen by Aristotle, or to intervening modal and epistemic operators.  §6 focuses on the problem of “borderline contradictions”: the range of acceptability judgments for apparently contradictory sentences with vague predicates as surveyed in empirical studies, and the theoretical implications of these studies. Finally, §7 surveys the ways of contradiction and its exploitation in literature and popular culture from Shakespeare to social media.