Cytokines (cyto:cell; kine:factor) are produced by cells and serve as chemical messengers for one type of intracellular communication. Cytokines play a central role in host defense mechanisms. Defense against infectious and tumoral disease depends on nonspecific myelomonocyte defenses in conjunction with specific immune processes. Both systems are regulated by various leukocytes in the blood and tissue. All these cell components are produced in the bone marrow from hematopoietic stem cells. Cytokines are soluble messengers allowing deployment and coordination of all cell systems. Despite the complexity of the cytokine network, we now have a better understanding of the interactions between the different components determining secretion and activity of these mediators. This knowledge may hold the promise of better control of immune and inflammatory responses. Experimental data shows that the cytokine balance can be modulated in auto-immune, immune deficiency, and malignant diseases, opening up new perspectives for therapy and perhaps vaccination.