A person's mood is a measurable affective state, which can consist of a combination of emotions. In normal functioning, moods are largely adaptive influenced by external events.

A mood disorder (such as clinical depression) is a pronounced maladaptive mood. Similarly, seasonal affective disorder describes an disorder where seasonal changes (particularly the length of the day) has an abnormally strong influence on mood.

An optimist and a pessimist evaluate a situation relatively favorably and unfavorably, respectively. This applies also to expectations. The optimist looks at the world "through rose-tinted spectacles," (or a slightly more modern adaptation, "through rose-colored [or lemon-colored] glasses"), i.e. with expectations of a favorable outcome, whereas a pessimist will tend to concentrate on the possibility of outcomes being unfavorable or unpleasant.

There is tentative evidence that the evaluation of situations is related to neurotransmitter concentrations in the brain. Different neurotransmitters appear to affect the processing of positive and negative outcomes, possibly by acting on different sub-systems within the brain (Source: "Why we do what we do", New Scientist, 31 July 2004).

Mood disorders are mental illnesses where the normal functioning of mood is disrupted. The most common mood disorders are clinical depression and bipolar disorder. It also seems likely the anxiety disorders are related to mood disorders. Drug therapies for these disorders appear to target neurotransmitter functioning within the brain.