Increasing evidence suggests that mindfulness and meditation positively influence social functioning and interpersonal relationships. Existing trait mindfulness scales, however, primarily assess mindfulness as it occurs outside interpersonal contexts. To more fully understand the relational benefits of mindfulness, the authors proposed the general construct of interpersonal mindfulness – mindfulness as it occurs during interpersonal interactions. This chapter describes the development and psychometric properties of the Interpersonal Mindfulness Scale (IMS). An initial pool of 49 items was subjected to exploratory factor analyses, and an iterative process of item deletion and item generation resulted in a final scale of 27 items. Confirmatory multigroup factor analyses across five independent samples (n = 2,085) revealed a hierarchical solution in which four first-order factors were components of a single higher-order interpersonal mindfulness factor. The four resulting subscales were named presence, awareness of self and others, nonjudgmental acceptance, and nonreactivity. Further psychometric tests, including Rasch analyses and generalizability theory analyses, showed good internal reliability and acceptable test-retest reliability of the IMS over 1-month periods. Construct validity was demonstrated from the convergent, discriminant, and incremental validity of the IMS using partial correlations, partialling out the shared variance of trait mindfulness with intrapersonal, interpersonal, and relationship functioning variables. Initial tests of the IMS have demonstrated strong reliability and validity, but future work will continue to improve the measure.