This chapter presents the research on the effects of pollinators on seed production by examining pollen limitation in natural plant populations. The sufficiency of natural pollination is usually tested by pollen addition experiments, whereby seed set of flowers that are hand pollinated with cross pollen is compared to seed set of flowers that are pollinated naturally. In pollen limitation studies, an increase in seed set following hand pollination may not necessarily result in an increase in plant fitness if trade-offs between fitness-related traits occur. Trade-offs between current reproduction and other fitness-related traits are commonly termed costs of reproduction. Many studies in natural plant populations have shown that resources limit seed production. Many studies have suggested that sequential flowering of sympatric species that share pollinators is the result of past selection to avoid pollen limitation caused by competition for pollination. In many plant communities there is substantial overlap in flowering times of species that share pollinators.