Sensory systems respond not only to stimuli from the environment but also to cues generated by an animal's own behaviour. This leads to problems of sensory processing because self-generated information can occur at the same time as external sensory information. However, in motor regions of the CNS corollary discharges are generated during behaviour. These signals are not used to generate movements directly but, instead, interact with the processing of self-generated sensory signals. Corollary discharges transiently modulate self-generated sensory responses and can prevent self-induced desensitization or help distinguish between self-generated and externally generated sensory information. Here, we review recent work that has identified corollary discharge pathways at different levels of the CNS in vertebrates and invertebrates.