We describe the evolution, over three phases, of Bronfenbrenner's theory from an ecological to a bioecological theory. Phase 1 (1973–1979) culminated in the publication of The Ecology of Human Development (1979). Phase 2 (1980–1993) saw almost immediate modifications to the theory, with more attention paid to the role of the individual and greater concern with developmental processes. In Phase 3 (1993–2006), proximal processes were defined and placed at the heart of bioecological theory, and from 1998, the Process-Person-Context-Time (PPCT) model was described as the theory's appropriate research design. Given the extent of these changes, and to avoid theoretical incoherence, scholars should be cautious about stating that their research is based on Bronfenbrenner's theory without specifying which version they are using.