The convergence of heterogeneous networks, such as cellular mobile systems, wireless local area networks and other wireless broadband systems, leading to the so-called beyond 3G networks, introduces an increasingly strong trend, to margin monolithic approaches in favor of more flexible wireless network architectures. Initially in this paper, the generic requirements for beyond 3G radio systems, based on heterogeneous networks are given. Various architectures for beyond 3G systems based on heterogeneous networks are described. Due to the diverse capabilities and nature, of each of the networks comprising such a heterogeneous system, QoS provisioning in such an environment is even more challenging than traditional monolithically built systems. These issues are raised in detail and a service-based approach aiming at simplifying the QoS provisioning, from the application, the terminal and the network management system’s perspective is provided. A comparison of the aforementioned architectures, with special focus on the QoS provisioning issues and requirements from the underlying networks is given and finally conclusions are being drawn.