In this chapter the main physical phenomena related to electromigration are described. First, the electromigration driving force as a result of the action of the ``wind force'' is derived. Next, the role of the several paths for atomic migration in an interconnect line is discussed and the material transport equations are presented. Several earlier continuum models of electromigration are then described. Here, the impact of mechanical stress is explained. The void nucleation condition is discussed in detail and, finally, the basic void evolution equations and associated numerical methods are briefly presented.