Starling's hypothesis ascribes fluid movements across capillary walls to the interaction of hydrostatic and colloid osmotic forces. For 90 years it has been recognized as the basis of plasma-to-interstitial fluid balance. Its original statement was based on the notion of capillary impermeability to plasma proteins. However, as knowledge of transcapillary exchange of plasma proteins developed, its formulation was progressively modified to allow for protein transport and for interaction of protein transport with volume flow. The most important aspects of the conceptual evolution of Starling's hypothesis are reviewed in the text of this lecture.