The development of the concept that aldose reductase (AR) is involved in diabetic complications is presented from its early beginning when Dr Van Heyningen first found polyols in sugar cataracts in 1959. The involvement of the polyol theory of sugar cataract is described. The prevention of sugar cataract formation by aldose reductase inhibitors dramatically demonstrates the role of this enzyme in sugar-induced cataracts. The possibility of AR involvement of other diabetic complications was an obvious extension of the polyol theory. The use of AR inhibitors in preventing diabetic changes in nerve and retina in animals strongly suggest that AR may play a role in the development of other diabetic complications.