Depression is a mental state characterised by a pessimistic sense of inadequacy, despondent lack of activity and sustained low mood. It can be seen as a dysfunction, adaptive defence mechanism or a mood disorder, typified by feelings of sadness, anxiety, worthlessness, guilt and irritability, manifesting in physical symptoms such as insomnia, disturbed sleep, waking early, fatigue, loss of energy, and aches, pains or digestive problems that are resistant to treatment.

Products such as the Carroll Depression Scales-Revised (CDS-R) can be used as screening instruments or repeat tests to measure progress during treatment.