The observation of a model is a necessary process in the context of modeling and simulation as it offers to modelers the results of their simulations. In this paper, we focus our works on the observation mechanism which is generally not explicit nor clearly specified. This is generally not an issue unless we want to use our model in experimental frames or to avoid the observation mechanism to interfere with the simulation. In this paper, we introduce an extension to the Parallel Discrete Event System Specification (PDEVS) formalism, to observe models in various ways, by event (at each state transition of a model), at the end of the simulation or by a time step.Thus, we define a formal specification of this extension and its abstract simulators algorithms. Finally, we present an implementation in the DEVS framework VLE.