This introductory chapter provides a background of how mass immunization programs made vaccination a cornerstone of Chinese public health and China a site of consummate biopower, or power over life. Over the twentieth century, through processes of increasing force, vaccines became medical technologies of governance that bound together the individual and the collective, authorities and citizens, and experts and the uneducated. These programs did not just transform public health in China—they helped shape the history of global health. The material and administrative systems of mass immunization on which these health campaigns relied had a longer history than the People's Republic of China itself. The Chinese Communist Party championed as its own invention and dramatically expanded immunization systems that largely predated 1949 and had originated with public health programs developed in southwestern China during the Second Sino-Japanese War from 1937 to 1945. The nationwide implementation of these systems in the 1950s relied on transformations in research, pharmaceutical manufacturing, and concepts of disease that had begun in the first decades of the twentieth century. These processes spanned multiple regime changes, decades of war, and diverse forms of foreign intervention. Most important, they brought with them new ideas about what it meant to be a citizen of China.