Neuronal circuits transform sensory information into behavior. I will present our recent work to understand the neuronal circuit basis of decision making and motor control. First, we studied synaptic connectivity underlying choice-selective activity in the mouse posterior parietal cortex (PPC) during a perceptual decision-making task. Using a combination of virtual-reality behavior, two-photon calcium imaging, connectomics, and circuit modeling, we analyzed how synaptic connections between PPC neurons relate to their function. Our results provide evidence for how synaptic connectivity in cortical circuits supports a learned decision-making task. Second, we examined the wiring logic of premotor circuits controlling the Drosophila leg and wing. Using connectomics, we found that premotor networks organize into modules linking motor neurons innervating muscles with related functions. By comparing the architectures of leg and wing motor control systems within the same animal, we identified common principles of premotor network organization and specialized features reflecting the biomechanical constraints and evolutionary origins of these motor control systems.