The law of mass conservation constrains the dynamics of interacting populations. These stoichiometric constraints facilitate and provide a unified framework for theoretical studies of species interactions such as competition for resources, allelopathy, commensalism, mutualism, and predation. In theory, stoichiometry affects conditions for the coexistence and stability of interacting populations, and generates predictions of population persistence in relation to nutrient supplies in a habitat. Experimental studies support many predictions of stoichiometric theory as applied to interacting populations. Stoichiometric signatures are also predicted for the assembly of diverse communities. Ecological stoichiometry provides important insights into the biological mechanisms behind population persistence and community diversity.