Ergodic theory is a very robust method to understand/distinguish dynamical systems. To a Riemannian manifold (M,g) we can associate a canonical dynamical system, its geodesic flow. During this talk we will discuss the ergodic theory of geodesic flow on negatively curved manifolds. We will prove results about entropy of this flow and explain how starting from that we can conclude information about isometry groups and connection to open problems.