This monograph deals with the theory of metastable and unstable states. Metastable states of matter are well known phenomena of nature. A familiar example is a supercooled vapor which with proper precautions can be maintained in a metastable state for quite a long time. Eventually, however, this vapor will condense into the more stable, equilibrium liquid phase. This condensation requires the occurrence of thermal fluctuations of density of a certain critical size (the "critical droplet"). A certain activation energy is required for the formation of such droplets.