X-ray diffraction (XRD) is an indispensable characterization method for epitaxial thin films. It is used to determine crystal structure, phase purity, film thickness, epitaxial strain, interfacial structure, defect formation, and compositional uniformity. A wide range of samples can be measured, including simple layers of uniform chemistry, compositionally graded films and heterostructures, superlattices, free-standing membranes, and even amorphous complex oxide layers. XRD is reliable, non-destructive, and is the most rapid way to obtain quantitative information about a thin film sample. In this chapter, we provide a guide to standard laboratory-based XRD for epitaxial oxide thin films. We begin by introducing the instrumentation and definitions of angles and relevant concepts. We then describe the commonly applied methods of X-ray reflectivity and high-resolution diffraction, as well as the more advanced techniques for determining strain (reciprocal space mapping), texture (phi scans and pole figures), and mosaicity (rocking curves). The approach in this chapter is to provide the user with a toolkit: we explain exactly how to set up the measurement, which experimental parameters are appropriate, and how to analyse the data. The techniques described herein are just the starting point for more complex and in-depth techniques described in the other chapters of this book.