The cognitive dimension of Borderline Personality Disorder has received relatively little attention in the clinical literature and is poorly understood. This article illustrates how a range of cognitive problems including attention deficit disorder and learning disabilities may contribute to the cognitive disturbances identified in the disorder including dissociation, paranoia, all or nothing thinking, overvalued ideas, and denial and splitting. A review of relevant research supporting the presence of cognitive deficits is summarized along with a developmental pathway for the expression of the cognitive dimension. Clinical examples are provided. Recommendations for inclusion of assessment and treatment strategies that address cognitive deficits within a psychodynamically based psychotherapy are discussed.