Recent efforts to improve and complete modulation theory are described, emphasizing factors that arise in attempts to understand the diffusion tensor based upon turbulence theory and the theory of charged particle scattering. Direct numerical solutions of the transport equations are used to study sensitivity of cosmic ray modulation to the correlation length. It is shown that modulation is strongly influenced by the radial variation of the correlation length. This variation is still poorly understood, in part owing to the uncertain impact of pickup ion driven turbulence in the outer heliosphere and to practical difficulties in measuring the parallel correlation length in the outer heliosphere.