For the control of the medfly, Ceratitis capitata, using the Sterile Insect Technique, special strains are now being used which produce only males. The use of these strains has had a major impact on the overall efficiency of this procedure by increasing significantly the amount of sterility induced in field populations. Genetic sexing strains are based on the use of male linked chromosomal translocations which enable selectable marker genes to be linked to the male sex. The principles behind the development of the strains will be described, together with an overview of the different selectable genes that have been tested and the stringent constraints placed on the choice of the appropriate translocation. Two selectable genes have been incorporated into functional sexing strains namely, white pupa (wp), which enables male and female pupae to be differentiated on the basis of colour and temperature sensitive lethal (tsl), which enables females to be killed by an increase in ambient temperature. The role of genetic recombination on the stability of the strains is extensively discussed as it impacts directly on the field application of the strains. A genetic analysis of mass reared strains revealed that instability could be caused by several factors which all had to be taken into account during selection of the appropriate translocation. Field testing of strains in cages and during small scale releases revealed no deleterious effects on competitiveness of the genetic manipulation. Large scale field releases of sterile flies clearly demonstrated increased effectiveness of an all male release compared to a bisexual release; this encouraged the uptake of the strains by SIT programme managers. Currently, the vast majority of operational medfly SIT programmes are using genetic sexing strains.