ABSTRACT. The Functional Mock-up Interface is a standard for co-simulation, which both defines and describes a set of C interfaces that a simulation unit, a Functional Mock-up Unit (FMU), must adhere to in order to participate in such a co-simulation. To avoid the effort of implementing the low level details of the C interface when developing an FMU, one can use the Overture tool and the language VDM-RT. VDM-RT is a VDM dialect used for modelling real-time and potentially distributed systems. By using the Overture extension, called Overture FMU, the VDM-RT dialect can be used to develop FMUs. This raises the abstraction level of the implementation language and avoids implementation details of the FMIinterface thereby supporting rapid prototyping of FMUs. Furthermore, it enables precise time detection of changes in outputs, as every expression and statement in VDM-RT is associated with a “timing cost”. The Overture FMU has been used in several industrial case studies, and this paper describes how the Overture toolwrapper FMU engages in a co-simulation in terms of architecture, synchronisation and execution. Furthermore, a small example is presented.