The generation and learning of movements involves interactions among specific neuronal populations within and across brain regions. Important among these regions, for movement among other functions, are the basal ganglia, a set of evolutionarily conserved interconnected deep-brain nuclei. This is strikingly illustrated by how their pathological dysfunction in Parkinson's disease deeply impacts the ability to move. The basal ganglia control the production of different movements through dedicated circuits. These span multiple brain nuclei with complex but highly organized entry and exit routes, from the striatum, the classical input nucleus, to the substantia nigra pars reticulata (SNr), one main output nucleus. Decades of research have focused on understanding the signals that are processed through basal ganglia circuitry and how they contribute to the control of movement. In this primer, we focus on direct interactions between basal ganglia and the brainstem, through which specific populations of basal ganglia output neurons communicate with select brainstem motor centers to influence descending circuits for action. Breakthroughs in understanding brainstem circuits for action execution have made it possible to map the organization and function of the interface between basal ganglia and brainstem. This combined work has led to deep insights into how basal ganglia regulate movement with great granularity.