Arthritis of the knee is commonly encountered in primary care and specialty settings. The initial work up and treatment of these patients requires a thorough evaluation and complete series of plain radiographs. Despite increased use of advanced imaging modalities, plain films remain the most instructive, available, and affordable diagnostic test. The evaluation of patients with arthritis can be challenging, by using a standardized approach one can perform a quick and adequately comprehensive evaluation. Here we outline an approach appropriate for the evaluation of these patients in any setting. Particularly, we advocate a complete history and physical exam augmented by a four view radiographic series: A weight bearing anterior-posterior (AP) of the knee, a 45 degree weight bearing posterior-anterior (PA) view of the knee, a standing lateral, and a Merchant view. Through our described systematic evaluation of these views, a great deal of information can be obtained often helping guide treatment plans created with the patient, and helping avoid a potential missed or delayed diagnosis. Background Osteoarthritis (OA) is the most common form of arthritis, and the knee is the principal large joint affected by this process [1]. In men and women over the age of fifty-five, an estimated ten percent develop disabling knee pain from OA; 25% of those affected are considered severely disabled [2]. The incidence of osteoarthritis of the knee increases with age, is strongly associated with obesity [3], and more commonly affects women [4]. OA is the single greatest contributor to disability [5,6], and in the elderly, its overall risk of disability is comparable to that of cardiovascular disease. The precise etiology, pathogenesis, and progression of OA are not yet completely understood. Taking a thorough history and performing a detailed physical examination is necessary when evaluating a patient with presumed osteoarthritis of the knee. The most common presenting symptom is pain, often associated with activity and relieved by rest. Additional symptoms include crepitus, swelling, limping, stiffness that typically improves after a brief period of activity [7], and progressive lower extremity deformity (e.g. bow-legged or knock-knee deformity). Patients should be asked about their occupation, history of trauma, and presence of previous knee pathology. Due to the fact that there is a 50% risk associated with genetic predisposition to OA, a careful family should be documented [8-10]. Physical exam may reveal genu valgum (knock-knee) or genu varum (bow-legged), an extensor mechanism lag, a leg-length discrepancy, or a flexion contracture. Arthritis is categorized as primary or secondary. Primary arthritis is an idiopathic condition that is thought to result from mechanical wear of the joint beyond the body’s reparative capabilities. In contrast, secondary arthritis is due to another condition or disease, such as trauma, autoimmune disorders, crystalline arthropathy, and congenital anomalies. The radiographic hallmarks of primary and secondary arthritis differ. Identifying these differences correctly can lead to significant adjustments in the approach to patient care and are one important aspect of radiographic evaluation of the patient with osteoarthritis. When evaluating both primary and secondary arthritis of the knee, weight-bearing radiographs from multiple views are essential to allow visualization of the functional joint space. The specific purpose of this review is to provide Internists, Physiatrists, Rheumatologists, and Orthopaedic Surgeons with a systematic method for evaluating radiographs to help guide appropriate treatment in the patient with osteoarthritis of the knee. Systematic Evaluation of the Arthritic Knee In the evaluation of the adult patient with degenerative joint