There is disclosed a motor vehicle electronic control unit casing. The casing includes a housing (1) for electronic components, and a cover (15). The housing and cover (15) are inter-engageable. The housing (1) has a base (2), and an upstanding peripheral side wall (3) defining a peripheral edge (4) in spaced relation to the base (2) and which extends around an access opening of the housing. The cover (15) has a closure part (16) and is configured to close the access opening via engagement between the closure part (16) and the housing (1) around the peripheral edge (4). The cover (15) also has a mounting portion configured to be mounted to a mounting surface (27) in a motor vehicle. The mounting portion depends from the closure part (16) so as to extend substantially alongside the peripheral sidewall of the housing (1) when the housing and the cover (15) are inter-engaged to close the access opening, the mounting portion thereby being configured to secure the housing relative to the mounting surface (27) such that the peripheral edge (4) is spaced from the mounting surface (27).