Why are there only 2 sexes? Why are sperm much smaller than eggs in most sexually reproducing organisms? In this talk, I will review the literature on the evolution of anisogamy. Theories for the evolution of anisogamy can be broadly divided into three categories-- those that emphasize the conflict between the sperm and egg strategies (and regard sperm as parasites), ones that emphasize the mechanisms of contact between eggs and sperm, and ones that stress on a uniparental inheritance of cytoplasmic DNA. Tests of these theories are often carried out on data from colonial green algal species, which show a range from isogamy to anisogamy.