A secure control unit is provided with a completely sealed casing (22) of which at least one wall (24) forms a heat sink. The circuit board provided in the casing (22) is designed such that all of the heat generating components (16-20) which require cooling are located on the lower side of the circuit board (10) and in thermal connection with the heat sink wall (24). Components which do not generate excessive heat are located on the upper side of the circuit board (10). Raised conductive blocks (32-36) couple thermally the components (16-20) to the wall (24). With this arrangement it is not necessary to provide cooling internally of the casing (22) and it is possible nevertheless to use high powered components of the type which are found in modern computers. The casing remains secure and thus suitable for high security applications.