A comprehensive review of the studies of gas-liquid two-phase flow patterns and flow-pattern maps at adiabatic and diabatic conditions is presented in this paper. Especially, besides other situations, this review addresses the studies on microscale channels, which are of great interest in recent years. First, a fundamental knowledge of two-phase flow patterns and their application background is briefly introduced. The features of two-phase flow patterns and flow-pattern maps at adiabatic and diabatic conditions are reviewed, including recent studies for ammonia, new refrigerants, and . Then, fundamental studies of gas-liquid flow patterns and flow-pattern maps are presented. In the experimental context, studies of flow patterns and flow-pattern maps in macro- and microscale channels, across tube bundles, at diabatic and adiabatic conditions, under microgravity and in complex channels are summarized. In addition, studies on highly viscous Newtonian fluids (non-Newtonian fluids are beyond the scope of this review) are also mentioned. In the theoretical context, modeling of flow-regime transitions, specific flow patterns, stability, and interfacial shear is reviewed. Next, flow-pattern-based heat transfer and pressure drop models and heat transfer models for specific flow patterns such as slug flow and annular flow are reviewed. Based on this review, recommendations for future research directions have been given.