There is provided a 3D printing build material container (1). The container comprises a collapsible reservoir, (3) a relatively rigid reinforcement structure (4), a build material outlet structure (13) and a gas inlet structure (14). The reservoir is to hold build material (11). The reinforcement structure is to resist collapsing of at least one reinforced portion of the reservoir. The outlet structure is to allow build material to exit the reservoir. The inlet structure is to allow a gas to enter the reservoir.