Species composition reflects a combination of environmental and historical events at a site; hence, changes in species composition can provide a sensitive measure of ecologically relevant changes in the environment. Here, we consider the analysis of species composition when multiple sites are followed through time. Analyses of temporal trends in species composition either summarize species composition into a few metrics (indices or axis scores) or analyze the similarity among sites. We develop and illustrate the similarity approach. Each pair of samples represents a pair of replicates, a pair from the same site at different times, a pair from different sites at the same time, or an unrelated pair. Differences among times can be estimated by comparing average temporal dissimilarity to average replicate dissimilarity. Temporal trends can be described by one of three statistics that measure progressive change, the correlation of temporal dissimilarity with the length of time between samples. These methods are illustrated using data on changes in a South Carolina zooplankton assemblage following disturbance, and changes in bird species composition on Skokholm Island, Wales. It is difficult to define and interpret temporal trends. Some definitions of interesting trends, like increasing divergence from another set of sample plots, place additional requirements on the sampling design. Including replicate samples or clustering sample plots and including “control” plots for comparison with sentinel sites would contribute to an understanding of changes in species composition.