A tyre monitor for mounting on a wheel rim, the tyre monitor comprising a housing having a first part (644A) configured to receive a component substrate (645) and a second part (644B) configured to receive one or more other components of the tyre monitor, for example a battery. The first and second parts of the housing are non-coplanar and non-parallel with one another. When the tyre monitor is mounted on the wheel rim, the substrate is non-coplanar and non-parallel with a plane (T) that is tangential to the wheel rim at the location where the tyre monitor is mounted on the rim and preferably the substrate plane is substantially radial with respect to the wheel rim.