A process for expanding or puffing fibrous or plastic materials such as paper, tobacco, cotton, asbestos, plastic films, etc., whereby the expandable material in sheet or filament form and a puffing agent are continuously advanced under pressure through a pressure zone defined between a pair of opposed surfaces which continuously separate at the zone outlet so that as the expandable material and puffing agent pass out of the pressure zone the agent expands and puffs the material. Heat is usually applied to the pressure zone to generate or increase the pressure therein. The puffing may be in the form of a liquid, such as water or fluorinated hydrocarbons, or a gas, such as steam. The puffing agent may be supplied separately to the pressure zone or concurrently with the expandable material.