Like eukaryotes, bacteria express receptor Ser/Thr protein kinases (STPKs) that initiate a wide variety of signaling networks. Recent biochemical and structural studies of the STPKs of Mycobacterium tuberculosis have revealed that bacterial and eukaryotic STPKs adopt common folds and share mechanisms of substrate recognition and regulation. Mycobacterial receptor STPKs are activated by dimerization through two distinct interfaces that promote activation-loop phosphorylation. The active STPKs phosphorylate diverse substrates within the bacterial cell, including other kinases as well as proteins involved in many central physiological processes. In the case of the FHA-domain protein, GarA, the unphosphorylated protein regulates primary metabolism, while phosphorylation mediates GarA autoinhibition. These studies have begun to define the activation mechanisms and the biological regulatory functions of the mycobacterial STPKs.