Certain examples of a build material container for a three-dimensional printing system is described. The build material container has an external casing and an internal compartment within the external casing. The internal compartment may have a reservoir to store build material, where the reservoir has a channel structure, and a surround that encloses the reservoir. A partition is provided within which the channel structure is mounted, the partition forming an upper surface of the internal compartment and being independent from the external casing and the surround. The partition restricts access to the internal compartment and collects build material during use of the channel structure.