A basic problem in marine biogeochemistry is understanding material and elemental distributions and fluxes in the oceans, and a key part of this problem is understanding the processes that affect particulate material in the ocean. Aggregation of particulate material is a primary process because it alters the transport properties of particulate material and provides a mechanism for transferring material from the dissolved into the particulate pools. Aggregation theory not only provides a framework for understanding these processes, but it also provides a means for making predictions and has been successfully used to predict maximum particle concentrations in the oceans and the fate of diatom blooms (including those from iron fertilization), the size spectra of particles in the oceans, and the size distributions of trace metals. Here we review the basic theory involved, summarize recent developments, and explore unresolved issues.