Extract: Complaints of impaired memory are amongst the most common symptoms voiced by patients to physicians in the fields of neurology, psychiatry, medicine, and surgery. Impairment of memory is one of the most disabling aspects of many neurological disorders, including neurodegenerative diseases, strokes, tumors, head trauma, hypoxia (reduced exposure of tissue to oxygen), cardiac surgery, malnutrition, attention deficit disorder, depression, anxiety, medication side-effects, and normal aging. This memory loss often impairs the patient's normal daily activities, profoundly affecting both the patients and their families. Research in memory began with neuropsychological studies of patients with focal brain lesions and now includes new methods such as PET (positron emission tomography, where the decay of an injected radioactive element or drug creates an image) and functional MRI (magnetic resonance imaging, where hydrogen atoms are polarized by a magnet and the summation of their spinning energy creates an image). Event-related methodologies have provided us with more refined and improved classification systems. Rather than conceptualizing memory as "short-term" and "long-term," we now think of memory as a collection of mental abilities that use different systems within the brain. In the present article we will summarize the four memory systems that are of clinical relevance: episodic memory, semantic memory, procedural memory, and working memory.