Some of the methods used in probing the structure and performance of heterogeneous catalysts are discussed. We consider X-ray diffraction, neutron diffraction and neutron inelastic scattering, high-resolution electron microscopy, electron diffraction, electron energy loss spectroscopy, and computational methods. The determination by X-ray diffraction of siting and occupancy of extra-framework cation Ni2+in faujasite or by neutron diffraction of H+in La3+-Y catalyst are discussed. Some of the methods are complementary and a combination of two or more can be used to gain a better understanding of the problem.