Efforts to prepare students for college, careers, and civic engagement have traditionally emphasized academic skills, but a growing body of research suggests that interpersonal and intrapersonal competencies, such as communication and resilience, are important predictors of postsecondary success and citizenship. One of the major challenges in designing educational interventions to support these outcomes is a lack of high-quality measures that could help educators, students, parents, and others understand how students perform and monitor their development over time. This report provides guidelines to promote thoughtful development of practical, high-quality measures of interpersonal and intrapersonal competencies that practitioners and policymakers can use to improve valued outcomes for students. Key findings include: (1) Five broad tasks must be accomplished to develop and implement appropriate measures of interpersonal and intrapersonal competencies: defining and selecting constructs, identifying the intended uses of the measure, developing measures, evaluating the technical quality of measures, and documenting consequences of assessment use; (2) To determine which competencies to address first, start by examining research to understand what measures currently exist across the domains of interest; how good they are from a technical, as well as a practical, perspective; and how difficult it is likely to be to develop new ones; and (3) Four kinds of development activities are likely to be needed: (a) conduct basic research to understand the nature of the psychological processes or behavioral manifestations that underlie a construct, (b) develop new measures for a construct that is well understood, (c) assess or improve the quality of an existing measure of a construct, or (d) investigate the consequences of using a measure in the school context. The following are appended: (1) Summary of the White House Workshop on Hard-to-Measure 21st-Century Skills; and (2) Experts Who Participated in Meetings and Interviews.