Since the early 1990s, the measurement of cytokines has become an important tool to understand mechanisms of immunotoxicity as well as to identify and classify xenobiotics. During the past decade, several major scientific and technical developments have greatly increased the efficiency of measurement. Most of the current attention is focused on methods that allow the simultaneous measurement of many cytokines, be at the messenger ribonucleic acid level (microarrays) or at the protein level (Luminex-based assays). Next, during the last decade quantitative polymerase chain reaction methods have gained widespread use. Finally, intracellular cytokine staining has provided detailed and invaluable information on the immune responses. This chapter describes the most widespread techniques for measuring cytokines. In addition, it tries to suggest which technique may be the optimal choice for specific purposes.