Abnormal psychology is the study of mental disorders – what they look like (symptoms), why they occur (etiology), how they are maintained, and what effect they have on people’s lives. Mental disorders are surprisingly common. For example, a study conducted by the World Health Organization examined the prevalence, or frequency, of mental disorders in people visiting medical doctors in primary care settings in 14 countries. Defining abnormality is deceptively difficult. When asked to describe abnormal behaviour, people typically say that it occurs infrequently, is odd or strange, is characterized by suffering, or is dangerous. All of these are reasonable answers for some types of abnormal behaviour, but none of them is sufficient in itself, and making them all necessary results in too strict a definition. One parsimonious and practical way to define abnormal behaviour is to ask whether the behaviour causes impairment in the person’s life. The more a behaviour gets in the way of successful functioning in an important domain of life, the more likely it is to be considered a sign of abnormality. For every student, teacher and researcher in the subject it offers a solid basis for an in-depth understanding of the entire subject area.