Heterogeneous Wireless Networks (HetNets) are a class of networks that have been designed to provide an open and flexible architecture for the coexistence of various distinct Radio Access Technologies (RATs). In such networks, one of the key challenges is the selection of the best available RATs when a user is connected upon making an incoming service request. This thesis proposes a Joint Call Admission Control (JCAC) - based approach for initial RAT selection in HetNets composed of two co-located wireless networks . The RAT selection problem in such HetNet is formulated by using a JCAC-based Semi-Markov Decision Process (SMDP) model, where the JCAC policy is determined according to a network cost function that weights three parameters: a cost of blocking the incoming service request, a cost of accessing RAT, and the RAT energy consumption cost. Simulation results are provided, showing the benefits of our approach in terms of RAT selection and resulting network energy consumption cost .