Osteoarthritis (OA) is the leading cause of musculoskeletal disability with limited treatment options. Although advanced imaging techniques are presently not applied in a clinical routine setting for diagnostic purposes, rapid technical developments in magnetic resonance imaging (MRI) and application to large longitudinal studies have contributed greatly to disease understanding in recent years. The focus of this presentation is semiquantitative (SQ) MRI assessment of osteoarthritis by expert radiologists, which has become a powerful research tool in understanding the natural history of OA. Several published reliable and validated scoring systems are available that have been applied to observational large cross-sectional and longitudinal epidemiologic studies as well as interventional clinical trials. Whole organ assessment of scoring different joint structures on MRI has shown adequate reliability, specificity, and sensitivity, as well as an ability to detect lesion progression. In comparison to quantitative and biochemical assessment of cartilage, SQ whole organ scoring analyzes all joint structures that are potentially relevant as surrogate outcome measures for interventional approaches. These include cartilage, the subchondral bone, osteophytes, intra- and periarticular ligaments, the menisci, the synovial lining, joint effusion and other periarticular structures such as cysts and bursae. MRI technology and sequence protocols have to be carefully considered when conducting OA studies applying SQ MRI assessment. Radiologists have to be aware of artifacts and differential diagnoses when using any of the SQ scoring systems. In an exemplary fashion the role of subchondral bone marrow lesions, synovitis and joint effusion in regard to the natural history and clinical outcome will be highlighted. Due to MRI a change in paradigm has occurred in that it seems accepted consensus now to perceive OA not as a disease of cartilage only but as a whole-organ disorder involving multiple joint tissues leading eventually to joint failure. Whole organ semiquantitative MRI assessment has contributed a lot to this change in perception and will be discussed in detail in this presentation.