This chapter explores perceptual disturbances, first distinguishing sensation from perception. It details the classical psychophysical methods (detection, discrimination, identification, and scaling) used to objectively measure perception in both human and animal subjects. This chapter then reviews the neuroanatomical foundations of key sensory systems, including vision, audition, somatosensation, and the chemical senses, emphasizing the role of the thalamus and association areas in mediating these processes. Perceptual disturbances are highlighted, including cortical blindness, agnosias, and amusia. Finally, this chapter focuses on the phenomenon of hallucinations. It discusses the various neuromechanisms underlying these disturbances and integrates serotonin, glutamate, and dopamine hypotheses of hallucinations.