Publisher Summary In the practical realization of gasification processes, a broad range of reactor types has been and continues to be used. For most purposes, these reactor types can be grouped into one of three categories, such as moving-bed gasifiers, fluid-bed gasifiers, and entrained-flow gasifiers. The gasifiers in each of these categories share certain characteristics that differentiate them from gasifiers in other categories. Moving-bed gasifiers (sometimes called fixed-bed gasifiers) are characterized by a bed in which the coal moves slowly downward under gravity as it is gasified, generally by a countercurrent blast. In such a countercurrent arrangement, the hot synthesis gas from the gasification zone is used to preheat and pyrolyse the downward flowing coal. With this process the oxygen consumption is very low, but pyrolysis products are present in the product synthesis gas. There are one or two processes that do not fit into any of these three main categories. These include in situ gasification of coal in the underground seam, as well as molten bath processes, which are described in this chapter.