This document provides an overview of disturbances in perception, including illusions and hallucinations. It defines illusions as misperceptions of real sensory stimuli, while hallucinations occur in the absence of stimuli. Various types of illusions and hallucinations are described based on etiology, sensory modality, and other factors. Both illusions and hallucinations can affect individuals without psychiatric disorders under unusual conditions like fatigue, sensory deprivation, or grief. The document also discusses other perceptual distortions and how perception is influenced by mood, anxiety, attention, and other psychological factors.