There is provided a 3D printing build material container (31) comprising a reinforcement structure (34), a collapsible reservoir (33) to hold build material, a build material outlet structure (313), at least one first inlet structure (314, 316), and a second inlet structure (315). The reinforcement structure encloses an airtight space and the reservoir is disposed within the reinforcement structure. The build material outlet (313) structure is to allow build material to exit the reservoir. The at least one first inlet structure (314, 316) is to allow recycled build material to enter the reservoir and to selectively allow a gas to enter the reservoir. The second inlet structure (315) is to selectively allow a gas to enter a space between the reservoir and the reinforcement structure.