The collected evidence of the Symposium illustrates that sufficient information is now available to enable initial broad definition of the skin surface ecosystems for the different animal classes. As may be expected more is known about the ecosystems of classes (mammal, fish) and genera (e.g. Bos, Salmo) of commercial interest. A notable exception is the skin surface of the bird, about which, despite the development of intensive poultry husbandry, surprisingly little is known. The work to date on the morphology, chemical nature and inhabitants of the different ecosystems, however, provides only limited data on the dynamic processes, actions and co-actions occurring within them and the interchange of inhabitants between them. In particular, the role of the surface micro-climates in the development, survival and transfer of micro-organisms has still to be determined. Our present knowledge is thus almost entirely limited to recognition of the basic elements of the skin surface ecosystems and there is still a paucity of data on the variation between species, individuals and body regions, some of which appear to have specialised functions or like the joints of arthropods may be more susceptible to colonisation and invasion. Nonetheless, although the definition of the various surface ecosystems has by no means been completed, a stage has been reached where identification, classification and comparison of the defence mechanisms operating at the skin surface of different classes of animals can be attempted.