Heterogeneous modeling is modeling using several modeling methods. Since many different modeling methods are used in different crafts, heterogeneous modeling is necessary to build a heterogeneous model of a system that takes the modeling habits of the designers into account. A model of computation is a formal description of the behavioral aspect of a modeling method. It is the set of rules that allows to compute the behavior of a system by composing the behaviors of its components. Heterogeneous modeling allows parts of the system to obey some rules while other parts obey other rules for the composition of their behaviors. Computing the behavior of a system which is modeled using several models of computation can be difficult if the meaning of each model of computation, and what happens at their boundary, is not well defined. We propose an execution model that provides a framework of primitive operations that allow to express how a model of computation is interpreted in order to compute the behavior of a model of a system. When models of computation are ``implemented'' in this execution model, it becomes possible to specify exactly what is the meaning of the joint use of several models of computation in the model of a system.