Minibrain-related kinase (Mirk)/dual-specificity tyrosine-regulated kinase (Dyrk)1B is one of the best functionally characterized members of the Dyrk/Minibrain family of dual-specificity kinases. Dyrk family kinases are highly conserved mediators of growth control and differentiation. Mirk is expressed at high levels in skeletal muscle; thus, most of the recent studies of Mirk have used myogenesis as a model system to explore the function of Mirk in a native physiological environment. These studies have revealed that Mirk is a multifunctional Ser/Thr kinase that plays a critical role in muscle differentiation by regulatory effects on motility, transcription, cell cycle progression, and cell survival. Mirk also is found at elevated levels in various solid tumors, where it seems to act as a tumor survival factor. This review summarizes the known regulators and functions of Mirk kinase and outlines opportunities for future studies of Mirk in the fields of muscle and tumor biology.