A review is made of the various models and basic equations of gas-liquid two-phase flows which are indispensable for accurate analyses of the phenomena. As the most detailed model, a local instantaneous representation of mass, momentum and energy conservation is shown. Then, averaging procedures for the various physical quantities and their derivatives of two-phase flow are described with a particular attention to the existence of the interface. Based on the local instantaneous formulation and averaging methods, the averaged formulations of basic equations of two-phase flow are stated. Two-types of formulations based on mixture models and two-fluid models are shown. The mixture model is further divided into homogeneous model, a slip model and a drift flux model depending upon the treatment of velocity differences between phases. Approximations, assumptions and limitations of applications of the basic equations based on each model are described.