<p>Migration is a defining process in mountain regions worldwide, reshaping lives, livelihoods, economies, and landscapes, yet its developmental impacts remain deeply contested. This study examines the scale, patterns, drivers, and implications of out-migration in the Uttarakhand Himalaya, with particular attention to its intersections with the broader agenda of sustainable mountain development. Drawing on two years of ethnographic fieldwork across all ten mountainous districts of Uttarakhand, the research combines complete household enumeration in ten villages with focus group discussions and key informant interviews. The survey data were analysed using descriptive statistics and a binary logistic regression model, while qualitative insights provided community perspectives on the drivers and impacts of out-migration in the region. The findings reveal stark altitudinal and district-level variations: low-lying, better-connected villages report the highest out-migration, while remote, high-altitude settlements show lower rates. Migration is strongly selective, dominated by working-age males, educated youth, socially advantaged groups, and individuals employed in the service sector, while disadvantaged groups such as Scheduled Castes and women face constrained mobility due to limited resources and restrictive gender norms. Although remittances are received, their scale is limited because of high rates of permanent out-migration and they are largely spent on household consumption, reducing their developmental potential. At the same time, mass out-migration has led to village depopulation, abandonment of agro-pastoral land, feminization of mountain agriculture, declining farm output, and rising human-wildlife conflict. Interpreted through Neo-classical migration theory, the Push-Pull Model, the Neo-Marxist perspective, and the Aspirations-Capabilities framework, the results show that migration embodies both agency and constraint, producing uneven outcomes across altitudes, districts, and social groups. Importantly, the study emphasizes that migration should not merely be viewed as a developmental problem but as a mechanism that, if strategically harnessed, can contribute to sustainable mountain development in the Indian Himalayan Region (IHR). Leveraging migration for the sustainable development requires channeling remittances into productive rural investments, supporting return migrants in entrepreneurship and knowledge transfer, and strengthening social remittances to improve education, governance, and climate resilience. The study concludes with region-specific policy recommendations that focus on short-term livelihood support and longer-term systemic reforms, aiming to align migration with the goals of sustainable mountain development in the Uttarakhand Himalaya in particular, and the IHR more broadly.</p>