Caenorhabditis elegans exhibits a repertoire of behaviors that can be studied by genetic, anatomical and pharmacological approaches. Defecation is one of the simpler behaviors, involving a small number of muscles, a couple of neurons and only one neurotransmitter. This simplicity enables the precise characterization of the cells and genes required for executing the behavior and has made the defecation behavior a powerful model for investigating the genetic basis of nervous system function, muscle differentiation, rhythmic behaviors and oscillatory calcium signaling, and the metabolic and environmental regulation of behavior. Our review highlights how the function of a system even this simple results from the integration of many aspects of an organism's biology and involves the action of diverse genes.