A seedling's chances of establishing successfully are likely to be affected by the quantity of metabolic reserves in the seed. Seed size is thought to evolve as a compromise between producing numerous smaller seeds, each with few resources, and fewer larger seeds, each with more resources. Seed size varies 1011-fold across plant species, so the compromise has been struck at very different levels. These basic ideas have been accepted for 50 years, and many studies have interpreted seed size differences between species by reference to larger seed size being adaptive under a variety of hazards. However, experimental tests of the benefits of large seed size in relation to particular hazards have been rare. More experiments are now being reported, but a consistent picture has yet to emerge. There is typically at least a 105-fold range of seed mass between species even within a single area, suggesting that much seed size variation is evolutionarily associated with other plant attributes.