The term renal replacement therapy incorporates three modalities that control or correct biochemical and fluid disturbances of renal failure. Peritoneal dialysis and renal transplantation are two forms of renal replacement therapy that are outside the remit of this article. This review focuses upon the third group which are blood-based and involve direct treatment of a patient's blood in a closed, extracorporeal circuit. They provide renal replacement for end-stage renal failure and during periods of severe acute kidney injury, and also for non-renal indications such as the management of drug overdoses. Blood-based renal replacement therapies are often loosely referred to as 'haemodialysis', although this is only one of a range of treatments. This article outlines the theory and practical applications of these treatments.