The shift from device and application towards service-orientated IT Management raises new questions that require concepts such as portfolio management, resource planning or mass customization for IT services. These concepts increase the complexity of IT Service Management and require additional tool support. Conceptual models are necessary in order to develop appropriate tools. The goal of our paper is to propose and validate a conceptual IT service model. We introduce the characteristics of IT services and analyze existing IT service models. A common IT service model is derived (theoretically) from the literature and validated through cases of IT service providers. These case studies from three German IT service providers also yield insights for further research.