This article explores the contribution of behavior therapy to the extension of psychotherapeutic notions and techniques into everyday life, focusing on the transatlantic trajectory of assertiveness training. It traces the history of this behavioral intervention into interindividual relations from its emergence as a treatment for anxiety in postwar United States to its importation into the French field of continuing professional training at the turn of the 1980s. To understand what traveled between countries and practical fields, I first consider the definition of assertiveness as a skill sitting halfway between passivity and aggressiveness, which developed in the United States along with its uses outside therapy. I relate the success and inflexions undergone by assertiveness training between the 1950s and the 1970s to theoretical and strategic innovations in behavioral therapy and psychology, as well as to the reception of political and social movements, especially the women's movement. This article also shows that what moved between countries, sectors, and target audiences was not only an understanding of assertiveness as a socially acceptable expression of feelings, needs, and wants, but also diagnostic and action scripts fueled by the "ferment" of the 1960s. From middle-class American women to French managers, the expanded applications of assertiveness training were justified by the rhetoric of tensions between role socialization and new expectations for self-fulfillment and efficiency. Following the behavioral deficit model emphasized in assertiveness training, increasing calls for self-expression and participation prescribed communication skill training and a reconfiguration of interpersonal relations, both in the private and the work sphere. (PsycInfo Database Record (c) 2023 APA, all rights reserved).