Phases are homogeneous regions of matter--gas, liquid, or solid--that can be analyzed using common tools such as pressure gauges, thermocouples, and chromatographs. In this chapter, phases are distinct homogeneous regions larger than 100 μm. The order of phase listing is by decreasing water concentration. For example, the listing order LW H V LHC means that hydrates (H) contain less water than the liquid water phase (L W), but more water than vapor (V), which in turn contains more water than liquid hydrocarbon (LHC). The Gibbs phase rule for nonreacting systems provides the most convenient method for determining how many intensive variables are important in phase equilibria. For example, when excess gas (excess so that its composition does not change) contacts water to form hydrates, there are three phases (P 3, namely LW H V) and two components (C 2, namely water and a gas of constant composition), so that F 1; only one intensive variable (either pressure, temperature, or one phase composition) is needed to define the system.