Many organizations provide information technology services, either to external or internal customers. They maintain software, operate information systems, manage and maintain workstations, networks or mainframes or provide contingency services. A widely used framework on IT service quality (at least in the Netherlands) is the Information Technology Infrastructure Library (ITIL), which seeks to publish a standard of service quality and best practices. However, it does not provide organizations with the methodology needed to assess and improve their service processes based on assessments. We propose an Information Technology Service Capability Maturity Model (IT Service CMM) that can be used to assess the maturity of IT service processes and identify directions for improvement. This IT Service CMM originates from our efforts to develop a quality improvement framework that was targeted at helping service organizations to improve service quality. Case studies which introduced parts of our framework into different organizations had mixed results. A service process capability maturity scale, similar to the Software CMM, allowed us to explain many of the differences observed. At the same time, the IT Service CMM suggests ways to improve the service process capability.