<p>Potential etiologies of hip pain, especially in younger patients with acetabular dysplasia and various impingement syndromes, or in older patients following arthroplasty can be subtle radiographically, and quantitative assessment on standardized imaging is often performed to confirm the diagnosis and help clinicians choose between conservative or surgical management. As hip preservation and joint replacement surgical techniques have become more sophisticated and widespread, the number of measurements reported in the literature to detect subtle abnormalities has greatly increased, and measurement techniques may vary between institutions. As a result, musculoskeletal radiologists may be less familiar with particular measurements requested by clinicians. This article is the first of a three-part series describing measurements of the lower extremity and focuses on the most common measurements used by hip preservationists and hip replacement surgeons, including the proper study on which to perform each measurement, the proper measurement technique and the normal value(s) based on current literature. Finally, the implications of abnormal values for each measurement are briefly discussed. The measurements are grouped by anatomy and pathology; those that are more commonly used clinically and reported in the literature are discussed in greater depth, while those that are less common or may be primarily used for research are described more briefly with imaging examples.</p>