Personality has in the past been considered to be stable, or at the very least, something that changes slowly over time. However, all clinicians can think of a case where there was personality change on a seemingly more rapid basis. Sometimes this is in relation to treatment, sometimes not. It is to this phenomenon that this chapter addresses itself. I propose the concept of a state personality disorder (SPD). An SPD would be analogous to the concept of state anxiety, a set of symptoms that appear under certain circumstances and that may remit, perhaps in rapid fashion. In state anxiety these are anxiety symptoms, whereas in state personality these would be symptoms we usually think of as being personality symptoms.