Abstract The origin and physical meaning of the scaling and homogeneity hypotheses in the theory of equilibrium critical phenomena are discussed. The purely thermodynamic critical-point exponent relations, and those that also involve the coherence length, are first derived from separate and apparently unrelated hypotheses. It is then shown how Kadanoff's scaling picture provides a unified point of view from which both sets of relations may be obtained. The connection between this scaling picture and the recent renormalization-group theory is mentioned briefly.