This chapter investigates the collaboration between multiple mobile operators for optimizing energy efficiency in cellular networks. Most of the works in the literature optimize the performance of a given cellular network, without considering the existence of mobile networks belonging to other operators. This leads to suboptimal results, compared to the case where the optimization of the joint performance of mobile networks of multiple operators is considered. However, incentives need to be created to allow multiple (and generally competing) operators to collaborate for the purpose of energy efficiency. Indeed, random collaboration can cause certain unfairness among cooperative operators. Therefore, additional parameters should be considered to perform a fair green networking between mobile operators. This chapter aims to provide answers for similar situations. We start by investigating the case of uniform cooperative mobile operators having the same objectives and we establish cooperation decision criteria based on derived roaming prices and operators’ profit gains. Afterwards, we consider the case of non-uniform operators where a green operator focuses on exploiting the infrastructure of non-green operators to achieve CO 2 emissions saving. A two-level Stackelberg game is formulated to optimize the utilities of both types of operators.