The Ras-Raf-MEK-ERK intracellular signaling cascade can be activated in response to a variety of extracellular stimuli. Growth factor binding to extracellular receptors results in activation of Ras, which in turn interacts with and activates Raf, leading to the phosphorylation of the dual specificity kinase MEK (MAP kinase kinase) on two distinct serine residues. MEK possesses a number of unique biochemical and biological features that make it an attractive target from an anticancer drug development perspective. The identification and subsequent testing of highly selective small molecule inhibitors of MEK have served to re-enforce the long held belief that the MEK/ERK module plays a critical role in controlling a number of cellular events that are critical to tumor cell growth and survival. We have witnessed advancement of the first MEK-targeted clinical drug candidate into clinical trials with the entry of CI-1040. The evaluation of sufficiently potent and selective MEK inhibitors in well-designed clinical trials is critical for ultimate validation of MEK as a molecular-based anticancer drug target.