How does the brain control behavior?  Which neurons and circuits are critical for choosing the right action and then executing it?  How is a complex behavior organized?  We use the fruit fly Drosophila to investigate these questions.  Combing the rigorous analysis of behavior from neuroethology with the power of fly genetics for neural circuit cracking, we are identifying neurons and circuits that constitute the computational building blocks of all brains.  I will present data on neurons controlling feeding, courting, and grooming, showing how our screening approach enables understanding of both the logic of behavior organization and its neural control.