This chapter briefly describes the fundamentals of micro-CT. The sections will briefly describe principles of micro-computed tomography, reconstruction to image, acquisition, and creation of acquisition data. In the following sections, micro-CT components, tube-based micro-CT systems, and synchrotron micro-CT techniques are covered. In the finals sections, X-ray fluorescence computed tomography (XFCT), phase-contrast micro-CT, and small-angle X-ray scattering systems are being discussed. Topics including artifacts found in actual systems, precision and accuracy of reconstructions, and challenges and speculations for the future are covered in the following chapter.