Affective instability, defined as repeated, rapid, and abrupt shifts in mood, is considered the core pathology in borderline personality disorder. The temporal pattern of affective instability can be best captured with the experience sampling method-longitudinal assessment of people's affective states as they occur in real time and in their natural environment. A review of the experience sampling studies published to date for borderline personality disorder suggests the following mood variability pattern: intense negative mood, more frequent and abrupt mood changes than healthy controls and patients with major depression, and partial triggering of affect by external events. The method also has great potential to investigate the links between affective instability and other psychological and behavioral correlates of the disorder, such as suicide, lack of self-esteem, and erratic behaviors. However, the method requires systematic study to determine best data collection designs and mathematical models of mood variability.