The mitogen‑ and stress‑activated kinases‑1 and ‑2 (MSK1 and MSK2) are ubiquitous serine/threonine kinases that mediate intracellular signal transduction by mitogen‑activated protein kinases (MAPKs) of the p38 and ERK1/2 families. The structure of MSK is complex, featuring a substrate‑phosphorylating N‑terminal kinase domain, a regulatory C‑terminal kinase domain as well as many regulatory phosphorylation sites and motifs. Here we review all aspects of the structure‑function and mechanism of activation of MSK. First, we summarize the numerous and diverse environmental stimuli, ranging from physiological signaling molecules to cellular stresses that activate ERK1/2, p38α or both to effect activation of MSK in cells. Next is described the series of sequential phosphorylation events that first activates the C‑terminal kinase domain and finally the N‑terminal kinase domain of MSK in an activation mechanism that can be divided into four stages. We proceed by detailing the activation mechanism from a 3‑dimensional perspective, focusing on the roles of each of the phosphorylation events. Throughout, parallels are drawn to similarities with activation mechanisms of the kinase super family in general and with kinases of the AGC and CaMK groups in particular, to which the N‑ and C‑terminal kinase domains, respectively, belong. Finally, topics like MSK interactions with MAPK docking sites, MSK substrate specificity, monitoring of MSK activity in cells and mutant MSK as research tools are discussed.