Abstract: A circuit assembly having a plurality of planar substrates 14 with modules and/or components 13 mounted thereon, has the substrates comprising at least parts of walls of substantially enclosed hollow structures 15, for example, the substrates comprising two opposing and parallel walls of each structure, the structures are mounted spaced apart within a container 10, and the arrangement is such that cooling fluid is to flow from a supply to the circuit assembly, and is to be discharged from the circuit assembly, the cooling fluid to flow between the regions within the container surrounding the structures and the interiors of at least some of the structures, via holes 26 in the walls of the structures, so that the cooling fluid can be supplied easily and efficiently to points within the circuit assembly where generated heat is required to be removed, the construction of the circuit assembly being flexible in this manner.