This book explores the conception, design, construction, use, and afterlife of ENIAC, the first general purpose digital electronic computer. ENIAC was created and tested at the University of Pennsylvania from 1943 to 1946, then used at the Ballistic Research Laboratory in Aberdeen, Maryland until 1955. Unlike most discussion of early computers, this book focuses on ways in which ENIAC was used, and the relationship of its design to computational practice, particularly its use between 1948 and 1950 to conduct the first computerized Monte Caro simulations for Los Alamos. ENIAC’s first team of operators were all women, and the book probes their contribution to the machine’s achievements and the development of computer programming practice. ENIAC’s users changed its hardware and transformed its configuration over time, so that it eventually became the first computer to execute a modern program, defined by the authors as one following the “modern code paradigm” introduced in John von Neumann’s seminal 1945 “First Draft of a Report on the EDVAC.” They draw on new archival evidence to document the development of this idea and its relationship to work on ENIAC. They also use ENIAC to probe the construction of historical memory, looking at ways in which a bitter succession of legal battles around patent rights shaped later perceptions.