Electrochemiluminescence (ECL) is the light emission from a luminophore excited by homogeneous electron transfer between electrochemically generated radical species. This chapter describes the theoretical fundamentals of ECL, luminophores, mechanisms of ECL generation, and the steps occurred for its development into a powerful electroanalytical technique, owing the orthogonal nature of electrochemical stimulus and light response, which give ECL outstanding sensitivity and resolution. Furthermore, ECL intrinsically surface-confined generation prompted its evolution to an imaging technique, which is the focus of this book.