Disturbances in memory are the most common problem in patients with an organic mental syndrome. Other patients with significant psychiatric disorders also often have difficulty with memory. So it is very important in the clinical practice of psychiatry to understand the biological and neurocognitive mechanisms of memory proessing, and to develop the assessment tools with which memory function can be evaluated reliably and validly. Moreover, memory researches provide an important viewpoint from which we can understand the pathophysiological mechanisms of major neuropsychiatric illnesses. This article focuses on our understanding of memory functions in clinical and neurobiological aspects. The relevant material will be presented in four parts : 1) terminologies needed in defining major stages of various types of memory processing : 2) neurochemical and neuroanatomical basis of memory processing : 3) brief bed-side screening tests and more comprehensive neuropsychological tests for the evaluation of memory function : 4) the characteristics of memory dysfunction in several major psychiatric illnesses.