Antisocial personality disorder (ASPD) and psychopathy are related but distinguishable conditions with long histories in the mental health field. Recent years have seen a shift toward viewing these diagnostic conditions as dimensional and multifaceted, as opposed to discrete and unitary. This chapter covers historic and contemporary conceptualizations of these conditions and current approaches to assessing each. The authors describe the new dimensional system for personality disorders in the 5th edition of the Diagnostic and Statistical Manual of Mental Disorders. In addition, they discuss the triarchic model, an integrative framework for clarifying similarities and differences between ASPD and psychopathy, and guiding etiological research on these conditions. In particular, they consider how the three constructs of the triarchic model – boldness, meanness, and disinhibition – relate to distinct biobehavioral systems and measures. The chapter concludes with suggestions for future research that can help to advance our understanding of ASPD and psychopathy, with a focus on multi-method assessments and targeted treatments.