The plasmasphere is the vast "doughnut-shaped" region of the magnetosphere that forms a cold thermal plasma cloud encircling the Earth, terminating abruptly at a radial distance of 30,000 km over a sharp discontinuity known as the plasmapause. This is the first monograph to describe the historical development of ideas concerning the plasmasphere by the pioneering researchers themselves. The monograph brings our picture of the plasmasphere up to date by presenting experimental and observational results of the past three decades, and mathematical and physical theories proposed to explain its formation. The volume will be invaluable for researchers in space physics and will also appeal to those interested in the history of science.