The main challenge in the development of future wireless communication systems is to provide users with a wide range of services across different radio access technologies through a single mobile terminal, while maintaining the quality of service requirements. Thus, the need for seamless interworking between heterogeneous wireless communication systems consisting of multiple radio access technologies and overlapping networks emerges. In this paper we address the main issues that arise while implementing the interworking mechanisms between two different radio access networks, with emphasis on WLAN and WMAN. An interworking mechanism of both technologies based on IEEE 802.21 framework is proposed and described in detail. The framework is designed so that it copes with the specific characteristics of both WLAN and WMAN.