Air vehicle subsystems are moving towards more electric actuation and more electric architectures. This has resulted in transient electrical power demands that require high fidelity simulations to accurately predict the peak system electrical loads. In addition, the high power electronics necessary to drive more electric system devices are generating additional heat loads, resulting in increasing demands on thermal management system performance. Consequently, integrated system simulations and the capability to optimize integrated electrical / thermal management systems is becoming necessary at earlier stages in the design cycle. LM Aeronautics has developed a MATLAB / Simulink / SimPowerSystems based toolbox for rapid modeling of thermal and electrical subsystems. Use of this toolbox for simulating and optimizing an integrated thermal / electrical subsystem is demonstrated.