The article discusses the processes that determine the composition and diversity of communities, focusing on whether community assembly leads to a single stable equilibrium or multiple stable equilibria. It highlights that community composition is influenced by environmental conditions and interspecific interactions, but the historical sequence of species invasions can also play a crucial role. The paper reviews evidence from both theoretical models and empirical studies, suggesting that single stable equilibria are more likely in systems with small regional species pools, high connectance, low productivity, and high disturbance, while multiple stable equilibria are more common in systems with large regional species pools, low connectance, high productivity, and low disturbance. The author also presents preliminary findings from an observational study of small pond communities, which support these predictions and show significant effects on community similarity and species richness. The debate over community assembly, rooted in the work of Clements and Gleason, is revisited, emphasizing the importance of understanding the mechanisms of community assembly for predicting species composition and diversity patterns.The article discusses the processes that determine the composition and diversity of communities, focusing on whether community assembly leads to a single stable equilibrium or multiple stable equilibria. It highlights that community composition is influenced by environmental conditions and interspecific interactions, but the historical sequence of species invasions can also play a crucial role. The paper reviews evidence from both theoretical models and empirical studies, suggesting that single stable equilibria are more likely in systems with small regional species pools, high connectance, low productivity, and high disturbance, while multiple stable equilibria are more common in systems with large regional species pools, low connectance, high productivity, and low disturbance. The author also presents preliminary findings from an observational study of small pond communities, which support these predictions and show significant effects on community similarity and species richness. The debate over community assembly, rooted in the work of Clements and Gleason, is revisited, emphasizing the importance of understanding the mechanisms of community assembly for predicting species composition and diversity patterns.