Essentialism refers to the view that entities possess intrinsic, stable properties that determine what they are. Tracing the genealogy of essentialism from classical metaphysics through modern science, this chapter shows how essentialist assumptions entered psychology through conceptions of human nature, biological essentialism, and individualist models of personhood and how they came to be applied to various psychological phenomena. The chapter further shows how efforts to move beyond individualist essentialism can generate cultural and social essentialisms when such collective identities are treated as coherent and determining forces. Contemporary debates are reviewed, including essentialism as a cognitive tendency, the distinction between natural kinds and human kinds, and reification as the treatment of analytic abstractions as concrete entities. Drawing on social constructionism and poststructuralism, the chapter outlines reflexive, process-oriented alternatives that approach psychological phenomena as historically and relationally constituted while remaining attentive to the risk of essentialization involved in naming and classification.