Cytokines are signaling molecules that coordinate cellular interactions in immune and hematopoietic systems. During the past 5 years many cytokines and their receptors have been identified and cloned. With a few exceptions, cytokine receptors do not contain any known signaling domains and therefore, in order to trigger a specific cellular response, new and unusual features of signaling pathways must be assumed. A major advance in the field has come with the discovery of new family of cytoplasmic protein tyrosine kinases that associate with occupied cytokine receptors and make them competent for intracellular signal generation. This review describes some general characteristics of cytokine signaling.