There are at least two ways of approaching the study of nature. One tactic is to investigate a habitat or any convenient local environment with the aim of producing an inventory of the type of organisms found there, preferably with some indication of their relative abundance or population densities. The second approach concentrates upon what the organisms are doing in the habitat, and might involve measuring attributes of groups of animals which tend to exhibit similar interactions or use the same resources ? such as predators or the animals which eat plant foliage ? but which are not necessarily closely related. In the first case we are asking a descriptive question: "Which organisms live in the habitat?" We label them using a scientific name, but without additional information this does not provide us with any understanding of the habitat. In the second case we ask: "What are the organisms doing?" ? a functional question. The two approaches are complementary, with the descriptive approach often preceding the functional one, but they are nonetheless distinct. The ecological literature exhibits a clear dichotomy between these approaches, and books tend to focus on either one or the other. If the main goal is to introduce the reader to descriptive studies, an identification manual (with keys and line drawings) would be the usual publication. If the functional approach is adopted, an introduction to ecological patterns and the processes bringing them about would be the subject of the text. Rather little is known about the ecology of seasonal tropical freshwaters in general, and Hong Kong's freshwaters in particular. Moreover, there has been no attempt to synthesize the material that is available in the primary scientific literature. This book outlines the main ecological patterns, both temporal and spatial, which characterize Hong Kong streams and rivers. Attempts are made to account for the observed patterns by recourse to characteristics of the populations that comprise freshwater communities, and the properties and interactions of the individual organisms constituting those populations. Attention is centred upon animals although they are not considered to the exclusion of plants. The intention here has not been to produce an identification guide; indeed, it is worth emphasizing that our taxonomic knowledge of Hong Kong running-water fauna is restricted, as reflected in this text by the use of letters or numbers to designate as yet undescribed species within genera.