By placing the party grassroots at the centre of its focus, Building Socialism presents an original account of the formative first two decades of the Soviet system. Assembled in a large network of primary party organisations (PPO), the Bolshevik rank-and-file was an army of activists made up of ordinary people. While far removed from the levers of power, they were nevertheless charged with promoting the Party's programme of revolutionary social transformation in their workplaces, neighbourhoods, and households. Their regular meetings, conferences and campaigns have generated a voluminous source base. This rich material provides a unique view of the practical manifestation of the Party's revolutionary mission and forms the basis of this insightful new narrative of how the Soviet republic functioned in the period from the end of the Russian Civil War in 1921 to its invasion by Nazi Germany in 1941.