The NATO Research and Technology Agency has set up the Modeling and Simulation Group (MSG) 058 with title "Conceptual Modeling for M&S" to produce guidance on conceptual modeling for modeling and simulation (M&S) development within NATO. The Task Group is comprised of CAN, ESP, NOR, NLD, ROM, SWE, TUR and USA. This paper introduces the group's mandate and reviews the state-of-the-art of doing conceptual modeling. Current M&S standards, such as DIS and HLA, have provided the first steps to composability, interoperability, and reuse, but they are not sufficient to support understanding simulation development and to facilitate semantic consistency at the conceptual level of consequent simulation products. Systematic conceptual modeling will help to improve common understanding between stakeholders that are involved in simulation development, composition, and use for the defense establishment. This will lead to better requirement specification; and it is key to the transformation of requirements to implemented simulations fit for their intended use. NMSG-058 is committed to clarify the process and results of "Conceptual Modeling", and to recommend best practice to generate, document, and use conceptual models. The group is studying the fundaments of conceptualization of existing and prospective realities; methodologies for generating, documenting, and sharing conceptualizations; correlation of conceptual modeling with simulation and software engineering processes; and relevant initiatives and technologies. The main deliverable is a guide on the conceptual modeling process and product, that will provide best practice instruction and serve the needs of relevant stakeholders. Furthermore, the group is committed to offering the results of its efforts as the basis for establishment of a SISO standard. Copyright © 2008, SISO, Inc.