One of the most fundamental issues in evolutionary biology is the nature of transitions from single cell organisms to multicellular ones, with accompanying cellular differentiation and specialization. Not surprisingly for microscopic life in fluid environments, many of the relevant physical considerations involve diffusion and mixing, for the efficient exchange of nutrients and metabolites with the environment is one of the most basic features of life. In this talk I will describe a synthesis of theoretical and experimental work that attempts to answer some basic questions about issues of transport important to multicellular life, using various model organisms to study the high Peclet number regime in which advection strongly dominates diffusion. Topics to be addressed include metabolic dynamics, phototaxis and flagellar synchronization in colonial alga and cytoplasmic streaming in aquatic plants.