Many mainframe installations struggle to understand their cost drivers in business terms. Which applications or business units drive which costs? How do changes to applications or business activity relate to increased costs? These questions often result in ambiguous answers because of the complexity involved in mapping technical measurements to business costs, applications, and organizations. Good data and tools can help manage the complexity. But successful cost allocation, whether it is used for chargeback, show-back, or just cost optimization, also requires careful planning. It impacts organization, processes, and mind-set. This presentation is based on real world experiences with business mapping and cost allocation from a wide range of installations including in-house, outsourced, and outsourcer environments.