Over the last several years there has been a dramatic shift in the role of amateurs, volunteers and hobbyists as it pertains to the production, distribution and use of information and communication technologies. The work presents four case studies that explore this transition and aim to explain how and why large-scale, participatory systems that are open to volunteer contribution are becoming important in our time. The cases include: (1) Linux, a free computer operating system, that is disrupting proprietary software models (2) BiOS, an initiative that aims to import participatory models into the life sciences; (3) the American Radio Relay League, a volunteer organization that connects radio hams in order to relay messages from coast to coast; and (4) the Ground Observer Corps, a cold-war paramilitary organization that uses volunteers as human radars for detecting enemy aircraft. The dissertation explores continuities and discontinuities between these systems and traditional information networks, develops the analytic term ?open systems?, and builds an explanatory framework that shows how relevant social groups who negotiate laws, norms, markets, and technical architectures, or code, effect the social construction of these systems. The work uses this framework to explore the clashes between the ideologies of openness and enclosure.