The purpose of a process heater is to heat some type of fluid, usually a liquid hydrocarbon. Process heaters, also called fired heaters, consist of the heater itself, the burners used to generate the heat, the process fluid being heated, and the controls for monitoring and adjusting the system. This chapter is not intended to be exhaustive as there are entire books written on the subject of fired heaters. Rather, it is intended to be representative with a particular focus on the fuel including a discussion of renewable fuels. Unlike many other industrial combustion systems, such as glass melters and steel reheat furnaces, the fuel composition in a process heater varies considerably. It is commonly a waste product from the production of, for example, gasoline, diesel, and jet fuel. The fuel variability is an important parameter that significantly impacts the equipment design, particularly the burners which need to operate safely on all fuels and efficiently with minimal emissions on the design fuels.  Some common applications for process heaters in those industries include distillation/ fractionation, thermal cracking, catalytic cracking, hydrotreating, hydrocracking, and catalytic reforming.