Dialysis fluid is produced by the blending of treated water with electrolytes at the patients bed side. Its preparation and composition are important elements of treatment optimisation since many of the constituents play a role in patient well-being. Ideally the composition of the dialysis fluid should match that of plasma, but due to differences between patients, as well as the increasing number of elderly patients receiving treatment, have resulted in a move towards individualisation of the electrolyte and buffer composition to patient needs. Such individualisation is facilitated by the availability of technology, however it is not yet possible to individualise minor electrolytes, such as K+, Ca2+ and Mg2+. Early dialysis treatments were frequently accompanied by pyrogen reactions arising from bacterial contamination of the dialysis fluid. Today the focus is on the stimulation of mononuclear cells by bacterial fragments contributing to chronic inflammation associated with long-term haemodialysis therapy, and which has led to suggestions regarding the desirability of using ultra-pure dialysis fluid to prevent or to delay complications associated with their presence.