Application of foams is widespread in both the food and plastic industries. The blowing agent or foaming agent is added to a formulation as the source of the gas used to produce a foamed structure. Two major types of blowing agents can be used for foaming: chemical blowing agents (CBAs) or physical blowing agents (PBAs). When CBAs are used, a gas is produced via a chemical reaction, while PBAs are introduced as a liquid or as supercritical fluid. This chapter focuses on the use of PBAs for foaming operations since these are of greater interest in pharmaceutical applications. During gas-assisted hot melt extrusion (HME) or foam extrusion (fHME), a gas is dissolved in the molten polymer- active pharmaceutical ingredient (API) system inside an extruder. Research on fHME has focused on the following areas: PBAs as fugitive plasticizers, foams to facilitate milling operations, and enhancing API release rate from solid dispersions.