Animal welfare and the potential for pain and distress to be experienced by animals used in science have concerned the general public and thoughtful researchers for a long time. It was these concerns, together with increasing use of animals in fundamental and applied research that motivated W.M.S. Russell and R.L. Burch to examine how decisions should be made about such use of animals. In the book The Principles of Humane Experimental Technique, first published in 1959, Russell and Burch proposed the concept of the Three Rs. The Three Rs stand for Replacement, Reduction and Refinement. Over the past 40 years the Three Rs have become widely accepted ethical principles, and are now embedded in the conduct of animal-based science in Canada and throughout many countries in the world.
Replacement alternatives refers to methods which avoid or replace the use of animals in an area where animals would otherwise have been used. This includes both absolute replacements (i.e. replacing animals with inanimate systems, such as computer programs) and relative replacements (i.e. replacing more sentient animals, such as vertebrates, with animals that current expert peer advice and interpretation of scientific evidence indicate have a significantly lower potential for pain perception, such as some invertebrates).
Reduction alternatives refers to any strategy that will result in fewer animals being used to obtain sufficient data to answer the research question, or in maximizing the information obtained per animal and thus potentially limiting or avoiding the subsequent use of additional animals, without compromising animal welfare.
Refinement alternatives refers to the modification of husbandry or experimental procedures to minimize pain and distress, and to enhance the welfare of an animal used in science from the time it is born until its death.
From the Canadian Council on Animal Care http://3rs.ccac.ca/en/about/three-rs.html