The immense success of wireless and broadband technologies over the last decade has been accompanied with an increasing number of wireless networks owned by different authorities. The main successful and attractive feature of these networks is the continued connectivity of mobile users, which has lead the network owners to tolerate increasing security issues. The challenged issues concern the authentication, the authorization and the accountability of users for fast roaming within and across networks. The research presented in this thesis is focused on authentication to enhance the secure seamless mobility of wireless users. I first provide a classification and an analysis of wireless authentication schemes. This investigation clarifies the drawbacks and uncovers aspects of the existing authentication schemes. Based on this knowledge, I provide a common authentication model for wireless technologies. Subsequently, the thesis presents authentication schemes for fast roaming in WLAN, authentication architectures for mobile groups across network, an Imbricated protocol suite for both, within and across networks authentication, and finally a scheme suitable for first responder mobile networks. All these proposals are based on the generic authentication model. (FSA 3) -- UCL, 2008