Modelling and Simulation as a Service (MSaaS) is a new approach being explored by NATO Science and Technology Organization (STO) Modelling & Simulation Group (MSG) Panel for a permanently available, flexible, service-based framework to provide more cost effective availability of Modelling and Simulation (M&S) products, data and processes to a large number of users on-demand. This Research Task Group is working on the development of the implementation of this framework, defining policies, stakeholders’ roles, services and reference architecture and reference engineering processes. MSaaS can be defined as “enterprise-based level architecture for discovery, orchestration, deployment, delivery and management of M&S services�. The University of Defence of the Czech Republic and the NATO M&S Centre of Excellence are investigating and proposing an approach to contribute to the definition of the MSaaS from an End User perspective. The paper proposes definition of M&S Software as a Service (MSSaaS), M&S Platform as a Service (MSPaaS) and M&S Infrastructure as a Service (MSIaaS) to introduce new roles and new business connections taking also into consideration Service Oriented Architecture (SOA) definitions and those definitions stated in NATO Modelling and Simulation Master Plan (NMSMP). In particular the authors propose a contribution to the definition of the different stakeholders’ roles and their relationships, starting from those of the MSG 136 group (M&S Group 136, Modelling and Simulation as a Service) and introducing new roles regarding the End User. In conclusion, this research and study activity proposes, in addition to the existing definitions, a taxonomy comparing roles across service models (MSSaaS, MSPaaS and MSIaaS). Furthermore, the M&S services’ classification is analysed in the framework of the MSG 136 Operational Concept draft, in order to identify the services which are to be properly composed and orchestrated to satisfy the End User requirements.