Cytokines are a heterogeneous group of polypeptide mediators that have been associated classically with activation of the immune system and inflammatory responses. An increasing number of related mediators is now included in this category and most of them have been shown to act on a variety of tissues, including the PNS and CNS. Cytokines and their receptors are expressed in tissues of these nervous systems, and might derive from invading immune, or resident, cells. Trauma in peripheral tissues might also induce cytokine-mediated events in the CNS, via either the circulation or secondary induction within the brain. In this first of a two-part review, the general properties, expression and recognition of these cytokines with respect to the nervous system are discussed.