Introduction: Motivation is an overlooked but crucial factor in determining whether solitude is psychologically beneficial or risky. This paper describes the development and validation of the Motivation for Solitude Scale -Short-Form (MSS-SF), a measure grounded in Self-Determination Theory that differentiates between intrinsic versus extrinsic motivations for solitude. Methods: Emerging adult (N = 803) and adolescent (N = 176) participants were recruited in four successive samples from the United States for the purposes of scale development and validation. Participants completed an on-line survey that included the MSS-SF and various well-being and personality measures.Results & conclusions: Confirmatory Factor Analyses resulted in a two-factor solution, selfdetermined solitude (SDS) and not self-determined solitude (NSDS), and showed the MSS-SF to be reliable with adolescents and emerging adults, with satisfactory convergent and discriminant validity. Engaging in solitude for extrinsic, not self-determined reasons was associated with loneliness, social anxiety, and depressive symptomatology; in contrast, solitude chosen for intrinsic, self-determined reasons was positively correlated with well-being, for emerging adults in particular. The MSS-SF goes beyond preference for solitude to distinguish two distinctly different motivations for solitude, and in so doing, allows researchers to better understand the affordances and risks of being alone for adolescents and emerging adults.The phenomenon of being alone is complex. One's aloneness can be experienced as isolation, loneliness, or solitude, and these are distinct constructs. Isolation is enforced solitude, sometimes used as a means of punishment such as solitary confinement, and is generally not a constructive experience of aloneness (Galanaki, 2005; Storr, 1988). Whereas isolation is an objective state of being separate from others, loneliness is subjective isolation, defined in the literature as a discrepancy between one's desired versus actual emotional or social relationships (Perlman & Peplau, 1982;Weiss, 1973). In contrast, solitude is defined as positive aloneness (Rubenstein & Shaver, 1982), "the constructive use of time alone," (Galanaki, 2013, p. 80), generally used for the purpose of engaging in intrinsically motivated activities (Moustakas, 1972). Despite the conceptual distinctions between these three states of being alone, research examining solitude has often conflated solitude with loneliness or isolation (Dahlberg, 2007;Gizatullina et al., 2016), or attributed the preference for solitude to social anxiety or shyness (Cheek & Buss, 1981;Leary, 1983), rather than to any positive or constructive motivation. This attribution contradicts theories positing solitude as a basic human need that spurs healthy psycholo-