The type of mating system we should use in any plan of livestock improvement requires very careful consideration. By reason of the particulate nature of inheritance and the behaviour of chromosomes in the cell divisions preceding the formation of eggs and sperm, the various mating systems differ in their influence upon the uniformity or otherwise of successive generations, the chances of securing improvement, the scope for control by selection, and finally, our ability to discriminate between the relative contributions of genetic and environmental variations to the population variance. Because of the genetic complexity many misconceptions have flourished about what we may expect with different mating systems, about the effects of inbreeding and the advantages and dangers of outbreeding. Different breeders have often secured different results with similar mating systems and the search for a rule of thumb guide has proved fruitless.