Abnormalities of mental state are frequently treated in psychiatry merely as symptoms that act as sign-posts pointing towards particular diagnostic conclusions. This chapter describes the mental phenomena prior to their becoming part of the formulation of particular disorders, but for convenience and coherence some common syndromes, such as mania, are used to draw together the associated phenomena. A hierarchy moving from feelings through emotions, moods, and affective state to temperament involves increasing complexity in terms of state of mind and usually to an increasing duration of that state. Delusion involves abnormal beliefs that arise in the context of disturbed judgements and an altered experience of reality. Depersonalisation and derealisation are assumed to arise from a disruption in the functions of consciousness to create amnesia, dissociative identity disorder and depersonalisation disorder. Speech disorder is usually separated from language and thought disorder.