We will discuss the basic formalisms of how semiconductor devices can be simulated, with a focus on advanced MOSFETs such as FinFETs, strain-engineered devices and non-Si channels. We will cover the assumptions and approximations that go into models, starting from the one-electron effective mass Schrodinger equation for envelope (wave) functions to calculate band structures. We will talk about the formalism of semi-classical transport, the Boltzmann transport equation and the method of moments that underpins the drift-diffusion and energy balance formalism, as well as Monte Carlo transport. We will conclude by discussing quantum transport approaches such as non-equilibrium Green's functions that are going to be increasingly important for nanoscale devices.