I will outline the physical mechanisms which produce the X-ray radiation we observed from the accretion flow in strong gravity. I will focus on how these can be used as a toolkit to interpret the spectra and variability of black hole and neutron star binaries and Active Galactic Nuclei. I also discuss how to use these to physically interpret real data using the publicly available XSPEC spectral fitting package, and how this has led to current models (and controversies) of the accretion flow.