The purpose of this work is to develop a differential Galois theory for differential equations admitting superposition laws. First, we characterize those differential equations in terms of Lie group actions, generalizing some classical results due to S. Lie. We call them Lie-Vessiot systems. Then, we develop a differential Galois theory for Lie-Vessiot systems both in the complex analytic and algebraic contexts. In the complex analytic context we give a theory that generalizes the tannakian approach to the classical Picard-Vessiot theory. In the algebraic case, we study differential equations...