This chapter contains sections titled: Why Fugacity? Fugacity Defined The Use of the Fugacity Pure Substance Fugacities Fugacities of Species in Mixtures Mixtures of Ideal Gases Why Ideal Solutions? Ideal Solutions Defined Why Activity and Activity Coefficients? Activity and Activity Coefficients Defined Fugacity Coefficient for Pure Gases and Gas Mixtures Estimating Fugacities of Individual Species in Gas Mixtures Liquid Fugacities from Vapor-Liquid Equilibrium Summary References