Liquid crystalline polymers are an important class of materials in which the existence of mesophases may control dramatically the physical properties. The phase behavior of liquid crystalline polymers is usually rather complicated, and many of these materials exhibit polymorphism and/or polymesomorphism. A combination of differential scanning calorimetry (DSC) and X-ray diffraction techniques is a very useful tool for the study of the phase behavior. Moreover, taking advantage of the extremely high intensity of synchrotron radiation, very short acquisition times can be used, so that the corresponding experiments can be performed under real-time conditions, and temperature programs similar to those employed in DSC can be imposed to the samples. In this chapter the application of synchrotron X-ray diffraction to the study of the phase behavior in liquid crystalline polymers is reviewed, with several examples in both main-chain and side-chain liquid crystalline polymers, analyzing the three scattering regions of interest: small-angle (SAXS), middle-angle (MAXS), and wide-angle X-ray scattering (WAXS), in real-time variable-temperature experiments.