The main international Information Technology Service Management (ITSM) process frameworks such as ITIL V3 and the ISO/IEC 20000-4, includes a service design process as part of their mandatory set of processes. Nevertheless, even with such availability of processes, their used nomenclature, their phase-activity structure, and their granularity level used for their descriptions, are non-standardized. Consequently ITSM academics are faced with a useful but disparate and diffused literature, and ITSM professionals lack of a suitable step-by-step service design methodologies. In this chapter, we present ITSDM (Information Technology Service Design Methodology), which is elaborated on best practices suggested in ISO/IEC 20000-4 and ITIL v3 design process-es with the aim to provide a step-by-step guideline with phases, activities, tasks, roles, controls and input-output artifacts. We illustrate its utilization with a real IT service (Cloud Storage service) that is ready operational in a German Higher Education Institution. We found that ITSDM provided qualitative benefits such as: ease of use, usefulness, compatibility and value. However, given the novelty of this methodology, we encourage for further empirical studies for providing more definitive results.