Borderline personality disorder (BPD) is a pervasive pattern of psychopathology characterized by unstable affect, suicidal behaviors, and identity problems (Diagnostic and statistical manual of mental disorders. Washington, DC: Author; 2000). BPD is heterogeneous in nature, highly prevalent in clinical settings, and increasingly studied by clinical and social psychologists. This review highlights affective instability, impulsivity, and interpersonal hypersensitivity, the interaction of which we believe accounts for the symptoms of BPD, the BPD criteria most associated with these features, and the interrelationships among these underlying dimensions. We also discuss difficulties in measurement of these dimensions. Real-world assessment methods will assist in the measurement of time-dependent processes and identify causes, covariates, or consequences of these processes to determine how these features manifest themselves in real life.