The concept of multimorbidity is still poorly understood and not well integrated into medical care and research. For clinicians involved in rheumatology care for an ageing patient population who have multiple diseases, multimorbidity is the rule not the exception. The interaction of different diseases and the impact they have on important clinical outcomes, such as physical function, quality of life and mortality, should all be considered by the rheumatologist. Treatment decisions must be adapted for the patient with multimorbidity to best serve the individual and society. This Perspectives article describes the concept of multimorbidity, how it differs from comorbidity, and outlines why an increased understanding of multimorbiditiy will enhance our overall clinical practice and research focus.