In this tutorial, a number of approaches to mathematical modeling of electrowetting-on-dielectric (EWOD), also known as digital microfluidics (DMF) will be reviewed. EWOD refers to methods for causing droplets to move along solid surfaces or changing the shapes of attached drops (e.g., to actuate a liquid lens) by applying a potential difference between the drop and an underlying electrode, separated from the conducting drop via a thin dielectric layer. The main equation describing electrowetting is known as the Young-Lippmann (YL) equation, which provides a relationship between the local contact angle of the drop and the square of the potential difference. In this tutorial, a simple derivation of the YL equation is provided based on an energy minimization principle. We will then introduce both lumped and field models to characterize the electrostatic forces acting on a drop as a function of its position relative to the underlying electrodes. The lumped model is based simply on treating the dielectric layer as a parallel-plate capacitor and considering the changes in the energy of the system as a function of the location of the drop. The field model requires the use of concepts from electromechanics, including Maxwell's electric stress tensor. We will consider both DC and AC electric potentials and describe how to analyze the system in both cases.