异常心理学又称变态心理学，是临床心理学的一个分支，针对各种心理疾病或异常行为描述其病理学及治疗方法，为更深入了解异常行为的入门学科，也是成为心理治疗师训练的必修知识之一。

从古至今，对未深入研究「变态」的一般人而言，「变态」往往代表着：与自己和周遭人的行为模式、思考模式或情绪反应模式极不相同，因为罕见而对现象不了解，因为不了解而恐惧，因为恐惧而试图逃避、排斥或强使对方改变成跟大多数人一样。

在过去，人们基本上以魔鬼论来解释变态心理。

在当代，变态心理学被分成心理异常和行为异常，这显示了哲学上关于身心问题和精神障碍分类的不同方法的双重性。“变态”有3个类别：低能，高才和天才。异常心理学的研究主要将人类心理和行为分为两种：适应和不适应的。「不适应」代表有问题存在，也代表着个人的脆弱和不易融入环境，而造成生活中各种他人不常发生的问题。变态心理学中「不适应」的定义是脱离常规，与大部分分人的行为，认知，和感受背道而驰。

Abnormal psychology is the branch of psychology that studies unusual patterns of behavior, emotion and thought, which may or may not be understood as precipitating a mental disorder. Although many behaviours could be considered as abnormal, this branch of psychology generally deals with behavior in a clinical context. There is a long history of attempts to understand and control behavior deemed to be aberrant or deviant (statistically, morally or in some other sense), and there is often cultural variation in the approach taken. The field of abnormal psychology identifies multiple causes for different conditions, employing diverse theories from the general field of psychology and elsewhere, and much still hinges on what exactly is meant by "abnormal". There has traditionally been a divide between psychological and biological explanations, reflecting a philosophical dualism in regard to the mind body problem. There have also been different approaches in trying to classify mental disorders. Abnormal includes three different categories; they are subnormal, supernormal and paranormal.