Species diversity occurs at different spatial scales and is a measure of biological variety at the species level of biological organization. Species diversity is widely used in monitoring ecological change and is often used in the form of an index. The Shannon–Wiener index is one of many indices of species diversity and is one based on the concept of evenness or equitability (i.e., the extent to which each species is represented among a sample). The origin of the Shannon–Wiener index is explained and the calculation of this index is illustrated with a simple worked example.