This critical review discusses the current understanding of the formation, transport, and merging of drops in microfluidics. The paper focuses on the physical ingredients that determine the flow of drops in microchannels and recalls classical results of fluid dynamics that help explain the observed behavior. It begins by introducing the main physical ingredients that differentiate droplet microfluidics from single-phase microfluidics, namely the modifications to the flow and pressure fields introduced by the presence of interfacial tension. Three practical aspects are studied in detail: (i) the formation of drops and the dominant interactions depending on the geometry in which they are formed; (ii) the transport of drops, namely the evaluation of drop velocity, the pressure-velocity relationships, and the flow field induced by the presence of the drop; and (iii) the fusion of two drops, including different methods of bridging the liquid film between them which enables their merging. The paper discusses the physical ingredients that differentiate droplet microfluidics from single-phase microfluidics, focusing on interfacial tension. It explains how interfacial tension can be thought of as a force per unit length and as an energy per unit area, and how it influences the shape and behavior of droplets. The paper also discusses the role of surfactants in modifying interfacial tension and the resulting effects on droplet behavior. The paper then discusses the production of droplets in microchannels, focusing on three main geometries: co-flowing streams, T-junctions, and flow-focusing devices. It explains the mechanisms of droplet formation in each geometry and the factors that influence the size and velocity of the droplets. The paper then discusses the transport of droplets in microchannels, focusing on the effects of interfacial tension, capillary number, and the geometry of the channel on the flow and velocity of the droplets. It also discusses the pressure drop and mean velocity of the droplets, and the effects of surfactants on these parameters. Finally, the paper discusses the merging of two drops, focusing on the mechanisms that enable the merging of droplets and the factors that influence the success of the merging process. It also discusses the effects of surfactants on the merging process and the role of interfacial tension in this process.This critical review discusses the current understanding of the formation, transport, and merging of drops in microfluidics. The paper focuses on the physical ingredients that determine the flow of drops in microchannels and recalls classical results of fluid dynamics that help explain the observed behavior. It begins by introducing the main physical ingredients that differentiate droplet microfluidics from single-phase microfluidics, namely the modifications to the flow and pressure fields introduced by the presence of interfacial tension. Three practical aspects are studied in detail: (i) the formation of drops and the dominant interactions depending on the geometry in which they are formed; (ii) the transport of drops, namely the evaluation of drop velocity, the pressure-velocity relationships, and the flow field induced by the presence of the drop; and (iii) the fusion of two drops, including different methods of bridging the liquid film between them which enables their merging. The paper discusses the physical ingredients that differentiate droplet microfluidics from single-phase microfluidics, focusing on interfacial tension. It explains how interfacial tension can be thought of as a force per unit length and as an energy per unit area, and how it influences the shape and behavior of droplets. The paper also discusses the role of surfactants in modifying interfacial tension and the resulting effects on droplet behavior. The paper then discusses the production of droplets in microchannels, focusing on three main geometries: co-flowing streams, T-junctions, and flow-focusing devices. It explains the mechanisms of droplet formation in each geometry and the factors that influence the size and velocity of the droplets. The paper then discusses the transport of droplets in microchannels, focusing on the effects of interfacial tension, capillary number, and the geometry of the channel on the flow and velocity of the droplets. It also discusses the pressure drop and mean velocity of the droplets, and the effects of surfactants on these parameters. Finally, the paper discusses the merging of two drops, focusing on the mechanisms that enable the merging of droplets and the factors that influence the success of the merging process. It also discusses the effects of surfactants on the merging process and the role of interfacial tension in this process.