The development and implementation of Microgrids have expanded the integration of distributed energy resources for economically and efficiently providing electricity and ancillary services to the main grid. This paper proposes a multi-objective mathematical model in order to optimally plan a Microgrid with ancillary services supplying capacity and positive revenue streams. The model is based on the size and location of distributed energy resources and the results show how the market influences the optimal design of the Microgrid, as well as the advantages of a multi-objective formulation.