The book is an historical ethnography covering the life course of the system from system emergence through 2017. It is based on archival research, surveys, interviews, and field work in four air traffic control facilities in the New England Region. Two examples demonstrate the agency of the workforce in maintaining the viability of the system. Incrementally, problem- solving people and organizations (management, union) inside the air traffic control system developed strategies of resilience, reliability, and redundancy that provided perennial dynamic flexibility to the system. Both examples demonstrate how the past manifests in the present. The first example demonstrates how, on September 11, controllers were able to clear the sky of over 4,000 airplanes in two hours and 15 minutes, an unprecedented, unrehearsed action performed without incident. The second shows the 2004-2017 intersection of two historical trajectories – modernization and a staffing crisis – that increased system risk. In response, controllers improvised tools of repair to adjust the liabilities of technological and organizational innovations to local conditions, contributing to safety and system persistence.