Psoriatic Arthritis is an inflammatory musculoskeletal disease. Its pathogenesis is still unclear. There is evidence that it results from multiple genetic and environmental factors. Approximately 30 % of the patients with psoriasis will develop the musculoskeletal manifestation in their lifetime. The real estimated number of cases is still unknown. Risk factors for the development of Psoriatic Arthritis in Psoriasis patients contain nail psoriasis. Different subtypes of Psoriatic Arthritis are differentiated. Those subtypes were used for classification of Psoriatic Arthritis in former days. In the meanwhile, it is well known that transitions of subtypes occur frequently. Psoriatic Arthritis can be discriminated from other types of arthritis using clinical characteristic, missing of Rheumatoid Factor and ACPA and imaging findings such as osteoproliferations and osteolysis. For classification of Psoriatic Arthritis the CASPAR-criteria are used nowadays. Due to the distinct patterns of Psoriatic Arthritis, including arthritis, enthesitis, dactylitis, and spondyloarthritis different possibilities for measurement of disease activity are available. The Synovio-Entheseal-complex, detected in animal model, is one of the theories for determination of PsA-origin, especially of the occurrence of the characteristic DIP-Arthritis. Due to the risk-population of Psoriasis patients, tools for early detection of Psoriatic Arthritis are important for clinical routine care for early treatment possibilities. Questionnaires as well as Imaging Biomarkers are developed for early diagnosis and still under examination to determine their use in clinical practice.