Small systems, such as clusters, polymers and other picoscale systems, exhibit forms clearly identifiable with the phases of bulk systems, such as liquid or one or more solid forms. However the thermodynamic behavior of these phase-like forms differs dramatically from the behavior of macroscopic phases. Phase equilibrium in small systems is a dynamic phenomenon, not the static equilibrium of macroscopic phases; phase coexistence occurs over bands of temperature and pressure, rather than only along sharp curves, e.g. of pressure and temperature, p(T); the phase rule loses its meaning; and many phase-like forms may occur for small systems that are unobservable in the macroscopic counterparts of those systems. Nevertheless the connection between the behavior of picoscale and macroscopic systems is straightforward, and the behavior of small systems is not difficult to understand. Furthermore, the behavior of small systems gives us new insights into the nature of bulk phase transitions.