The Uruguayan electric power system is facing deep challenges in the near future. The plans for 2015 include a significant expansion of generation based on renewable sources (wind, biomass) and a new HVDC international interconnection. These plans and the opening to multiple, distributed private generators pose significant challenges to the system operation and planning. This work describes the modal analysis of the Uruguayan Power System by considering a base 2004 scenario and two 2010 scenarios. These scenarios include conventional-thermal and hydro synchronous generation-and constitute a reference for a set of ongoing studies associated with the new expansion plans. The system comprises a relatively small electrical network with important hydroelectrical generation and highly interconnected with the neighboring countries. The main oscillation modes, which exhibit a poorly damped behavior, are analyzed in detail. The results were validated through simulations of the systems's response for different contingencies. The study includes the selection of appropriate machines to damp the oscillations and the tuning of Power System Stabilizers, PSS. The effects of this control action are then assessed via the modal analysis and validated via complete non linear simulation of transient response.