A comprehensive account of the properties of plasma loops, the fundamental structural elements of the solar corona. Plasma loops cover a wide range of sizes and range in temperature from tens of thousands to millions of degrees. They not only define the structure of individual active regions but connect different active regions--even across the solar equator. Loops also play an integral and decisive role in the enormous solar explosions called flares. Over recent years a wealth of space and ground-based observations of loops has been obtained in various widely-spaced regions of the...