The term electrowetting is commonly used for phenomena where shape and wetting behavior of liquid droplets are changed by the application of electric ﬁelds. We develop and analyze a model for electrowetting that combines the Navier–Stokes system for ﬂuid ﬂow, a phase-ﬁeld model of Cahn–Hilliard type for the movement of the interface, a charge transport equation, and the potential equation of electrostatics. The model is derived with the help of a variational principle due to Onsager and conservation laws. A modiﬁcation of the model with the Stokes system instead of the Navier– Stokes system is also presented. The existence of weak solutions is proved for several cases in two and three space dimensions, either with non-degenerate or with degenerate electric conductivity vanishing in the droplet exterior. Some numerical examples in two space dimensions illustrate the applicability of the model.