The Customer is the only non-developer role in eXtreme Programming (XP). The Customer's explicit responsibilities are to drive the project, providing project requirements (user stories) and quality control (acceptance testing): unfortunately the customer must also shoulder a number of implicit responsibilities including liaison with external project stakeholders, especially project funders, clients, and end users, while maintaining the trust of both the development team and the wider business. In this paper, we report on a series of case studies of the Customer role in XP projects. We have found that Customers have a pressured and stressful role, leading to issues of sustainability.