Biodiversity can be considered to be a human appreciation of the biological entity diversity. Diversity can be expressed numerically on the basis of taxa found, but it can also be expressed as the contribution of a specimen to the diversity, for which a formula is proposed. Diversity is the sum of the taxonomic or numerical diversity, and the ecological, genetical, historical, and phylogenetic diversity. Moreover, each group, or larger taxon, has its own characteristic diversity. These types of diversity are considered separately. One formula is tentatively proposed to calculate the overall diversity. Diversity in nature is variable, which is demonstrated in a diagram for comparison with societyinduced diversity changes. Diversity in the pelagic fluctuates differently from biomass fluctuations, but biomass fluctuations also influence diversity fluctuations. From a scientific and an environmental preservation point of view, special studies should be devoted to the Indo-Malayan neritic and oceanic waters, the neritic temperate and subtropical waters, the tropical waters in general, and especially to the Atlantic and Pacific Central waters.