This paper deals with the problem of characterizing and parametrizing the manifold of trifocal tensors that describe the geometry of three views like the fundamental matrix characterizes the geometry of two. The paper contains two new results. First a new, simpler, set of algebraic constraints that characterizes the set of trifocal tensors is presented. Second, we give a new parametrization of the trifocal tensor based upon those constraints which is also simpler than previously known parametrizations. Some preliminary experimental results of the use of these constraints and parametrization to estimate the trifocal tensor from image correspondences are presented.