Abstract: This review is divided into three parts. First, several problems in studying mechanisms of species coexistence in ecological communities are generalized. Second, some concepts explaining plant and animal species coexistence are reviewed. Within plant communities, these concepts include environmental factors, seed re-immigration, niche differentiation, balanced competition, species-pool, regeneration niche, coexistence of ecologically similar species and the null community. In animal communities, the effects of patch, edge, congregation, dispersion, interference and response to environmental heterogeneity are discussed. Finally, coexistence of specialists and generalists are discussed.