The catalytic activity of protein kinases is usually tightly controlled by posttranslational modifications and diverse sets of regulatory proteins. Protein kinases are highly dynamic enzymes, and structures of kinases in various activation states and costructures with regulatory proteins have provided critical insights into the complex regulatory mechanisms of this large and diverse protein family. The crystal structure of protein kinase A (PKA) provided a reference model for our understanding of kinase catalytic function. Now, more than two decades later, the high-resolution model of a full-length tetrameric PKA holoenzyme has been published, revealing the structural mechanisms underlying allosteric PKA activation.