This volume reports on the major experimental results arising from the NATO Experimental Workshop on the Size Structure and Metabolism of the Marine Microbial Food Web. It contains a collection of contributions related to the theme of size structure and metabolic function of pelagic ecosystems. The aim of this volume is to further the discussion on the potentials and limitations of the size based approach in plankton ecology. The editors and authors suggest that ecological approaches based on size, taxonomic or functional composition are complementary to one another.