Although the aetiology of rheumatoid arthritis(RA) is still unclear, accumulating evidences are clarifying the involvement of genetic and environmental factors in the disease, or the pathogenic mechanism of perpetuating proliferative synovitis and bone destruction. Genetic factors including HLA-DR, PADI4, death receptor 3 or DEK were reported, whereas low concordance rate of RA in monozygotic twins or other evidences strongly imply the involvement of environmental factors in RA. Each constituents of RA synovium including macrophages, stromal cells, T cells and B cells, is shown to be involved in the aetiology of RA. Recently osteoclasts are shown to be mainly involved in the bone-destruction of RA. Clarification of RA aetiology will provide alternative in RA treatment especially for refractory RA.