European industry has not been able to improve productivity in the area of simulator engineering, mainly due to the lack of standards and of a reference architecture. With the productivity goal in mind, ESA funded several initiatives to improve the simulator development process culminating in two projects aiming at defining a Reference Architecture for the development of Spacecraft Simulators. This paper will provide an overview of those two activities. In particular it will focus on the activity conducted at ESA/ESOC concerning Operational Simulators. The Reference Architecture for Operational Spacecraft Simulators is based on the guidelines set by ETM-10-21 and will be implemented using the upcoming European Simulation Standard ECSS E-40-07. It will be composed of Requirements and Interface Specification and the definition of a Development Process that will capture and formalise the way Operational Simulators are built. It will also provide a series of test cases that will be used to validate the defined interfaces. These test cases will be used by future simulators to perform testing using a common validation infrastructure.