Liquid fuels are widely used in various combustion systems for their ease of transport and storage. Due to their high energy content, liquid fuels are the most common fuels in transportation applications. Before combustion can take place, liquid fuel must be vaporized and mixed with the oxidizer. To achieve fast vaporization, liquid fuel is injected into the oxidizer (normally air) at high speeds. Soon after injection, the liquid fuel breaks up into droplets, forming a spray. Droplets then collide and coalesce, producing droplets of different sizes. Due to the high density of liquid fuel, the momentum of the liquid spray has a profound impact on local flow fields, creating turbulence and gas entrainment. In piston engines, the complexities of droplet combustion are further complicated by the occurrence of successive multiple transient events including gasification, ignition, flame propagation, and, ultimately, burn-out. As such, droplets can be considered the building block for providing fuel vapor in combustion systems. Understanding of single-droplet evaporation and combustion processes therefore provides important guidance in design of practical burners. Topics covered in this chapter include (1) droplet evaporation in both quiescent and convective environments, (2) droplet combustion, (3) initial droplet heating, and (4) characterization of droplet distributions.