Human perception is amazingly efficient and usually stable despite the intrinsic ambiguity and uncertainty of sensory input. However, occasionally our sensory systems do fail to reach a single interpretation for a given input. Sometimes, our percepts fluctuate between two distinct interpretations or states, although more than two interpretations are also possible. This is called bistable (or multistable for more than two states) perception. The perceptual alternations are spontaneous and stochastic, and the alternating percepts are often mutually exclusive. This entry describes varieties of bistable perception, binocular rivalry, mechanisms for perceptual switching, and tools used for studying mechanisms of object perception and visual awareness.