Willem Kolff designed his "kunstmatige nier" in the early 1940s using spare parts obtained from the Wehrmacht; with it, he treated 14 patients with acute renal failure. Although there has been a tremendous improvement in the design and construction of dialysis machines, the basic concepts are unchanged. In this review we show that dialysis dose and adequacy can now be predicted using simple clinical methodology. The second part of the article discusses the accumulation or excess removal of important biologically active substances which can result in hitherto unseen clinical syndromes and even pose a threat to life.