Scattering is one of the most fundamental interactions between light and inhomogeneous matter, often accompanied by changes in the amplitude, phase, frequency, and polarization of the light field. Label-free optical microscopy imaging techniques characterize the structural information of objects by measuring changes in these properties of the light field before and after scattering, offering noninvasive and damage-free imaging capabilities. Optical diffraction tomography (ODT) is an emerging label-free imaging method that combines quantitative phase imaging techniques with diffraction tomography theory. It employs diffraction tomography algorithms to process scattered light fields under varying illumination angles, reconstructing the three-dimensional refractive index distribution of objects quantitatively. Optical diffraction tomography can detect refractive index variations at the nanoscale and having previously been playing a significant role in cell biology and pathological diagnosis. Due to its nondestructive nature and capability of mapping three-dimensional refractive index distribution, ODT also holds great promise for characterizing materials and devices. In this chapter, the fundamental principle of ODT is reviewed, and the details of mathematic derivations and instrumental implementations are presented. Applications of ODT for characterizing photonic and optical data storage chips will be demonstrated. In the final part of this chapter, potential technical developments toward more complexed materials and practical application scenarios will be discussed.