In 1959, William Russell and Rex Burch published "The Principles of Humane Experimental Technique". They proposed that if animals were to be used in experiments, every effort should be made to Replace them with non-sentient alternatives, to Reduce to a minimum the number of animals used, and to Refine experiments which used animals so that they caused the minimum pain and distress. These guiding principles, the "3 Rs" of animal research, were initially given little attention. Gradually, however, they have become established as essential considerations when animals are used in research. They have influenced new legislation aimed at controlling the use of experimental animals, and in the United Kingdom they have become formally incorporated into the Animal (Scientific) Procedures Act. The three principles, of Replacement, Reduction and Refinement, have also proven to be an area of common ground for research workers who use animals, and those who oppose their use. Scientists, who accept the need to use animals in some experiments, would also agree that it would be preferable not to use animals. If animals were to be used, as few as possible should be used and they should experience a minimum of pain or distress. Many of those who oppose animal experimentation, would also agree that until animal experimentation is stopped, Russell and Burch's 3Rs provide a means to improve animal welfare. It has also been recognised that adoption of the 3Rs can improve the quality of science. Appropriately designed experiments that minimise variation, provide standardised optimum conditions of animals care and minimise unnecessary stress or pain, often yield better more reliable data. Despite the progress made as a result of attention to these principles, several major problems have been identified. When replacing animals with alternative methods, it has often proven difficult to formally validate the alternative. This has proven a particular problem in regulatory toxicology, especially when combined with the labyrinthine processes of the various regulatory authorities. The principle of Reduction would appear less contentious, but its application has highlighted the difficulties of providing appropriate expert statistical advice, especially in academic research facilities. In some instances, concern to implement Reduction strategies can result in the use of too few animals, which leads to inconclusive results, and wasteful experiments. It is in the area of Refinement, however, that major problems have arisen. Much of our judgement of what represents Refinement is based on little more than common sense. We make assumptions about animals and their feelings that often have little scientific basis. In many instances we may be correct, but these assumptions may become incorporated into institutional or national policies, without any attempt to verify them. To give an example - it is reasonable to assume that animals will experience pain after a surgical procedure, so pain-relieving drugs should be given to prevent this. We have some idea of the appropriate dose of analgesics for most animals, but effective pain relief requires that dose given is adjusted to meet the requirements of the individual animal. Requiring every animal to have the same dose of the same drug after any surgical procedure is not the best way of dealing with post-operative pain. Discussion of these problems should not detract from the very significant progress that has been made in the 40 or so years since Russell and Burch set out their guiding principles. What is needed now is greater academic focus on this area, not only to work on new methods of implementing the 3Rs, but also to disseminate current "Best Practice", and to revise this advice as further progress is made.