While the central nervous system has long been considered immunologically privileged, over the past decade it has become evident that a wide variety of leukocyte types traffic through the nervous system. It is also apparent that the rules governing the trafficking of these disparate cell types are different for each. Some arrive, and probably depart, continuously as part of normal physiology. Others only appear to seek a specific antigen or in response to tissue damage. In this review the nature and function of individual cell types are discussed and our current knowledge regarding the parameters governing their entry into the CNS is examined.