This chapter describes how synchrotron diffraction can be used to measure octahedral rotations within ABO3 perovskite oxide heterostructures. The collective rotations of the BO6 octahedra displace the oxygen atoms from the face-centered positions, effectively doubling the perovskite unit cell. As a result, a series of half-order reciprocal lattice points arise, which can be measured from oxide films using synchrotron X-rays. The presence and absence of specific rotations is indicative of the rotation pattern and thus provides a direct means to determine the crystal structure of the thin film. Through quantitative analysis of the half-order peak intensities, the average position of the oxygen atoms can be refined in strained heterostructures, including ultrathin films and superlattices. The chapter provides step-by-step details regarding how to carry out this approach, including discussion of what peaks to measure, how to calculate peak intensities, and how to extract atomic positions through error-minimizing fits of measured and calculated intensities. Numerous examples are provided to demonstrate the described approach as well as to illustrate how these measurements can be used to provide insights into epitaxial arrangements, physical properties, and dynamic responses of perovskite films to electric fields and optical excitation.