The introduction of new therapeutic agents in recent years has led the early diagnosis of rheumatoid arthritis (RA) increasingly important. Therapeutic intervention early in the disease course of RA allows earlier disease control and less joint damage. The 1987 American College of Rheumatology classification criteria for RA, however, does not reach full efficiency for providing clinicians to distinguish RA from other arthritides in the early phase of the disease. Therefore, diagnostic tools are needed to establish a reliable diagnostic criteria although the lack of an independent gold standard for RA makes it challenging in the evaluation of diagnostic studies. RA has no disease specific clinical, radiological or immunological features. This review describes the diagnostic values of medical history, physical examination, laboratory tests and imaging studies as well as the 1987 American College of Rheumatology classification criteria in the diagnosis of RA in its early phase. (Korean J Med 76:7-11, 2009)