The regulatory and commercial framework for the introduction of a cellular radio system in the UK is unusual, in that it calls for two competing networks to interwork interchangeably with a range of mobile units. Connection to the fixed telephone network is also required. The Total Access Communication System (TACS) has been defined to meet such conditions. A description is given of the bases for the determination of the overall system parameters, particularly those concerned with the radio paths and end-to-end speech transmission. The definition of the key interfaces to permit interworking without restricting competitive technological development is also outlined.