Organic photovoltaics (OPVs) belong to a class of devices where the nanometer scale morphology of the active layer has a large impact on device performance. However, characterization of the morphology of organic semiconductor mixtures that make up the active layer of OPVs remains a challenge. Here, the characterization methods that can be used to quantitatively and qualitatively measure the mesoscopic structure of the active layer in organic solar cells are described. Specifically, we focus on the use of X-ray and neutron scattering, scanning probe microscopy, and electron and X-ray microscopy for morphological characterization of organic semiconductor mixtures at mesoscopic length scales. © 2012 Elsevier B.V.