The forth generation (4G) of mobile communication networks will provide a multitude of real-time and non-real-time services on top of an IP-based platform. Subscribers can access these services over arbitrary radio-access technologies, including existing WPAN, WLAN, WMAN, and WWAN technologies as well as future air interfaces for broadband wireless access. Thereby, 4G networks pose high demands onto mobility management. This paper provides a comprehensive overview on existing mobility management approaches, identifying, classifying, and comparing the features of the solution space.