Phytoplankton exhibit a wide range of cell sizes and shapes. While the role of size has received much attention and theoretical analysis, the role of shape remains largely unexplored. Phytoplankton are often modeled as spheres, given that only a single dimension (the 'size') is needed to describe this shape. Unfortunately, spheres are not an average shape, but rather an extreme shape; spheres have the lowest surface area (and average cross-sectional area) to volume ratio of all convex shapes. A survey of phytoplankton shapes reveals that a sphere is not the prevailing form across the spectrum of cell sizes. In general, the degree of deviation from sphericity increases with increase in cell volume, but even for small cell sizes spheroids are commonly found. Phytoplankton shape is thought to affect many aspects of phytoplankton ecology and physiology including: light absorption, nutrient acquisition, predator-prey interactions, sinking and/or swimming. Effects of shape on each of these ecological functions have been examined in the past but a synthesis of the relative effect of shape on all of these functions has not been attempted. Here we present a systematic analysis of the effects of deviation from sphericity on all the above cell functions. This analysis is essential for developing better understanding of selective pressures on phytoplankton.Karp-Boss, L., E. Boss, and P.A. Jumars, 2005. The Role of Cell Shape in Phytoplankton Ecology. A Pilgrimage Through Global Aquatic Sciences, Santiago de Compostela, Spain, June 19-24.