This chapter discusses the effects of an electric field on the electronic and optical properties of semiconductor superlattices. Here an ideal superlattice is considered to be an infinite periodic series of strongly coupled quantum wells. The electric field is applied in the direction perpendicular to the layer planes. From a stationary point of view the field reduces the interwell coupling and localizes the electronic states into a finite number of periods. At large fields this leads to the splitting of superlattice minibands into Wannier-Stark ladders. From a dynamic point of view electrons in the localized states describe very fast oscillations, called Bloch oscillations. These effects directly influence the static and dynamic optical properties of semiconductor superlattices producing interesting electro-optic effects.