The biological disease-modifying antirheumatic drug abatacept (Orencia) has a novel mechanism of action; its activity is mediated via the selective modulation of T cell co-stimulation. This article reviews the clinical efficacy and tolerability of intravenous and subcutaneous abatacept in patients with rheumatoid arthritis (RA) and intravenous abatacept in patients with juvenile idiopathic arthritis (JIA), as well as summarizing its pharmacological properties. In patients with RA, the beneficial effects of intravenous or subcutaneous abatacept on signs and symptoms, disease activity, the progression of structural damage, physical function and/or health-related quality of life were seen in a number of well-designed trials, including in methotrexate-naive patients with early RA and poor prognostic factors and in patients with established RA and an inadequate response to either methotrexate or anti-tumour necrosis factor therapy. Subcutaneous abatacept plus methotrexate was also noninferior to subcutaneous adalimumab plus methotrexate in patients with active RA who were naive to biological therapy and had an inadequate response to methotrexate. In paediatric patients with JIA, intravenous abatacept improved signs and symptoms and delayed the time to flare. Abatacept was generally well tolerated in RA and JIA and was associated with low rates of immunogenicity. In conclusion, abatacept is an important option for use in the treatment of RA and JIA.