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If only good coverage is required in GSM networks, high frequency reuse factors can be used. In that case the network capacity is directly determined by the number of available time slots, and no advanced radio control algorithms are needed. If high capacity is required and fractional loading is used in GSM networks, advanced radio resource management algorithms are required. Sophisticated network simulators are needed for algorithm development in GSM base station systems. Also, finding out the capacity for accurate network planning is difficult without radio network simulators. In UMTS the need for an advanced radio network simulator will further increase because of new services, higher bit rates, multiplexing of services, and because of the possible asymmetric capacity requirement between uplink and downlink. Additionally, the UTRA WCDMA air interface is more dynamic than the GSM air interface, and therefore, optimised radio resource management algorithms need to be developed to fully exploit the WCDMA capabilities. The algorithm development for highly loaded dynamic networks is not possible without accurate modelling in computer simulations. In this paper the modelling principles of a WCDMA radio network simulator are presented.