This chapter presents optical measurement techniques aimed at fuel jets in compression ignition engines or in high-pressure-ratio gas turbines. Some basic studies that are related to rocket engines and a few examples from materials processing will be discussed as well. This chapter is meant to cover optical methods for fuel sprays that are undergoing a transition in thermodynamic state (sometimes called “transcritical jets”). It begins with some fairly well established techniques, followed by newer, more advanced techniques. Innovative combustion modes for high-efficiency internal combustion (IC) engines include partially premixed combustion (PPC) for compression ignition [1](sometimes spark assisted) and reactivity controlled compression ignition (RCCI, a dual-fuel system)[2]. These new modes achieve high efficiency in part because they operate at relatively high compression ratios. With a similar highefficiency goal, commercially available gas turbines are driving to higher pressure ratios. All of these powerplant concepts rely upon injection of liquid fuel directly into the combustion chamber, and the combustion modes of interest are typically based on fuel lean conditions. In addition, the IC engine modes rely upon very high levels of exhaust gas recirculation (EGR). Distribution of fuel vapor to achieve complete combustion with low emissions, within the necessary time frame and under these chamber conditions, is a challenge. The process of fuel/air mixture preparation via the spray thus controls combustion stability, efficiency, and production of pollutants. There is a direct relationship between the spray and high-efficiency, clean combustion modes. The conventional paradigm for fuel injection begins with ejection of an intact stream of liquid fuel from a nozzle into an air-filled combustion chamber. Spray formation occurs when the surface of the stream breaks up, driven by turbulence, shear, cavitation, or other instabilities, ultimately producing primary drops. These primary drops can break up further or collide and coalesce in regions of high drop density. The jet and the primary drops carry significant momentum, causing the