Fundamental failure mechanisms of electrical overstress/electrostatic discharge (EOS/ESD) and the physics behind them are the focus of this tutorial. Topics include the primary thermal failure mechanisms: junction burnout, oxide punch-through, and metallization burnout. Particular emphasis will be placed on the concept of simulation fidelity, which is crucial in the design of meaningful and robust ESD tests. Simulation fidelity is obtained by considering the interplay of the stress environment with the failure mechanism. Because this approach is developed for arbitrary stress environments, the considerations are equally applicable to EOS environments