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The chapter describes different patches that are included in a single landscape unit. Patches differ in size, species composition, and the way they function in a landscape. Patches may be composed of a single plant (one species), a group of plants (several species), or soil modified by the activities of animals. Mosaics are repeating units of several patch types across a landscape. Mosaics are characterized by clumped distributions of plant species created by endogenous factors such as the tendency of one species to create favorable or unfavorable microsites for other species or by exogenous factors such as the differential germination and the survival of individuals in distinct microenvironments. The structure of a patch dominated by a single plant species or produced by an animal species may be a function of landscape position. Position on a landscape, soil characteristics, and relationship to other patches are important variables determining the biological composition of patches. The structure of patches may also reflect competitive interactions. In deserts, where many of the organisms are living at or very near the threshold for surviving the climatic extremes, the availability of resources in patches is a critical variable. Competition for those limiting resources with other organisms is probably an important process determining the species composition and characteristics of a patch.