People differ in their social, emotional, and behavioral (SEB) skills: their capacities to maintain social relationships, regulate emotions, and manage goal- and learning-directed behaviors. In five studies using data from seven independent samples (N = 6,309), we address three key questions about the nature, structure, assessment, and outcomes of SEB skills. First, how can SEB skills be defined and distinguished from other kinds of psychological constructs, such as personality traits? We propose that SEB skills represent how someone is capable of thinking, feeling, and behaving when the situation calls for it, whereas traits represent how someone tends to think, feel, and behave averaged across situations. Second, how can specific SEB skills be organized within broader domains? We find that many skill facets can be organized within five major domains representing Social Engagement, Cooperation, Self-Management, Emotional Resilience, and Innovation Skills. Third, how should SEB skills be measured? We develop and validate the Behavioral, Emotional, and Social Skills Inventory (BESSI) to measure individuals' capacity to enact specific behaviors representing 32 skill facets. We then use the BESSI to investigate the nomological network of SEB skills. We show that both skill domains and facets converge in conceptually meaningful ways with socioemotional competencies, character and developmental strengths, and personality traits, and predict consequential outcomes including academic achievement and engagement, occupational interests, social relationships, and well-being. We believe that this work provides the most comprehensive model currently available for conceptualizing SEB skills, as well as the most psychometrically robust tool available for assessing them.