The thesis focuses on the implementation process of the IT Infrastructure Library (ITIL), the most widely used IT service management framework. ITIL consists of a broad variety of guidelines to standardize IT service management processes based on applied best practices, however ITIL itself gives no guidance on how to implement these practices. The benefits, critical success factors and barriers of ITIL implementation have been identified by multiple researchers, yet only an initial proposal has been made on the actual implementation process. This thesis aims to advance the work in developing an ITIL implementation process both in theoretical and practical terms. The theoretical framework is built around the initial 12 step implementation process model, which has not been tested previously. The theoretical framework further strengthens the process by connecting it to existing research in the area of ITIL, general and IT specific implementation theory, also proposing extensions to the model. Having developed a framework well grounded in theory, the practical validation of the process is advanced through a case study. The case study examines the relevance and applicability of the proposed implementation model with the help of interviews conducted at organizations which are planning to, or have already implemented ITIL. Based on the data collected from service management experts in different roles at the studied organizations, the development and validation of the implementation process is advanced in a real world setting. The findings underline the importance of selection and measurement of progress at different levels of the implementation, also stressing on the iterative nature of implementation processes. While confirming the validity of these generic process implementation principles in the ITIL context, also multiple ITIL specific nuances of the implementation have been explored. Altogether the proposed model is found to be relevant as a generically applicable guideline for ITIL implementation, outlining that no detailed, universally applicable recipe can be constructed. Instead, each organization must customize the proposed implementation process to their specificities.