Mueller Matrix polarimetry is a powerful optical technique for the characterization of both anisotropic and bianisotropic materials. This review emphasizes methods for the interpretation of measured Mueller matrices, from the meanings of matrix symmetries, to ab initio calculations of Mueller matrices that begin with Maxwell's equations operating on materials with permittivity, permeability and magnetoelectric constitutive tensors. We present an overview of polarimetry measurements in crystals as well as metamaterials that have optically responsive features on the order of the wavelength of visible light. Examples of the full measurement of the constitutive tensors of bianisotropic media from the analysis of their Mueller matrix, collected either in transmission or reflection, are illustrated for natural c rystals, polycrystals and nanofabricated metamaterials. Experimental and theoretical research into the complex optical responses of bianisotropic materials with polarized light is best expressed in terms of the Mueller matrix, as it offers an unambiguous and mathematically robust platform for analysis of light-matter interactions.