This paper presents results of NATO activities in the area of “M&S as a Service� (MSaaS). It illustrates potential benefits with regards to quality, efficiency, and interoperability that may be achieved by MSaaS and provides an insight how some of the existing challenges are currently being addressed. As M&S products are highly valuable to NATO and military organizations it is essential that M&S products, data and processes are conveniently accessible to a large number of users as often as possible. This requires a new “M&S ecosystem� that has to support stand-alone use as well as integration of multiple simulated and real systems into a unified simulation environment whenever the need arises. Due to many factors, service-based approaches are considered to be very promising for realizing future simulation environments. This idea is known as “Modeling & Simulation as a Service� (MSaaS). NATO Modeling and Simulation Group 131 (“Modelling and Simulation as a Service: New concepts and Service Oriented Architectures�) has investigated the idea of “M&S as a Service� as a 1-year Specialist Team. MSG-131 defined a consistent MSaaS terminology and placed MSaaS into the wider context of the NATO C3 Classification Taxonomy. Second, an exhaustive overview about service-based approaches used in the M&S domain in NATO and Partner Nations was produced. Third, a comprehensive overview of existing service-oriented (reference) architectures in the M&S domain was produced. A more detailed investigation of MSaaS and first steps towards an incremental implementation of a “Federated M&S Eco-System� are objectives of MSG-136 (“Modelling and Simulation as a Service - Rapid deployment of interoperable and credible simulation environments�) which started its 3-year term in November 2014. This paper presents results of MSG-131 and current work done by MSG-136.