This chapter concerns Batesian mimicry, which is the resemblance of a palatable species to an unpalatable or otherwise unprofitable species. Often these unprofitable models have warning signals, which the mimic has evolved to copy. The chapter also considers another well-known form of deception, namely masquerade, which is the resemblance of a palatable species to the cues of an object of no inherent interest to a potential predator such as leaves, thorns, sticks, stones, or bird droppings. Batesian mimicry and masquerade share many properties, and both can be considered examples of ‘protective deceptive mimicry’. We begin by briefly reviewing some well-known examples of protective deceptive mimicry. We then compare and contrast the various theories that have been proposed to understand them. Next, we examine the evidence for the phenomenon and its predicted properties, and finally we address several important questions and controversies, many of which remain only partly resolved.