The paper summarizes the principle of optimum diversity of biosystems, which suggests that biodiversity is a parameter to be optimized. In fact, diversity is considered as a major adaptation of biosystems to environmental conditions. Paper discusses the diversity of two hierarchical levels - population (phenotypic diversity within a population) and coenotic (number of species in community). Nypotheses about mechanisms of diversity optimisation in the ecological, microevolutionary and evolutionary processes and findings for environmental management and biodiversity conservation are considered.