This paper reviews the various factors that facilitate the high biodiversity of soil communities, concentrating on soil animals. It considers the problems facing soil ecologists in the study of soil communities and identifies the important role such communities play in terrestrial ecosystems. The review also considers diversity and abundance patterns. A range of factors are identified that may contribute to the biodiversity of soil and their role is reviewed. These include diversity of food resources and trophic specialization, habitat favourableness, habitat heterogeneity in space and time, scale and spatial extent of the habitat, niche dynamics and resource partitioning, productivity, disturbance and aggregation.Biodiversity of soil organisms appears high, largely attributable to the nested set of ecological worlds in the soil - the relationship between the range of size groupings of soil organisms relative to the spatial heterogeneity perceived by these various groups - that provide a large 'area for life' for the micro- and mesofauna. The role of aggregation and how it relates to the spatial scale under consideration and to species interactions amongst soil animals is largely unknown at present. The role of disturbance is equivocal and man's activities more often than not seem to lead to a reduced biodiversity of soil communities. This paper also identifies areas where further work is desirable to improve our understanding of the structure and functioning of soil communities.