This chapter investigates in detail the actual low-level migration processes. A solid grounding in the physics of electromigration (EM) and its specific effects on the interconnect will give us the knowledge to establish effective mitigation methods during the design of integrated circuits. We first explain the physical causes of EM (Sect. 2.1) and then present options to quantify the EM process (Sect. 2.2), which enable us to effectively characterize key aspects of the process and its effects. In Sect. 2.3, we introduce EM-influencing factors arising from the specific circuit technology, the environment, and the design. We then investigate detailed EM mechanisms with regard to circuit materials, frequencies, and mechanical stresses (Sect. 2.4). Since EM is closely related to thermal and stress migration that also occur in the conductors of electronic circuits, we examine their interdependencies (Sect. 2.5). Finally, Sect. 2.6 outlines the principles of a migration analysis through simulation. This honors the importance of finite element modeling (using the finite element method, FEM) in EM analysis and enables the reader to develop and apply similar modeling and simulation techniques.