Although almost all eukaryotic organisms have two sexes ? females and males ? the way in which the sexes are genetically determined varies enormously between species. In some organisms males carry two different sex chromosomes (XY-system, e.g. mammals), in others the females (ZW-system, e.g. birds), whereas the difference between both sexes can also be due to the copy number of individual chromosomes, such as under haplodiploidy e.g. insects), which is the main focus of our research. Besides these differences at the chromosomal level there is a large variation in the number and type of genes involved in sex determination. Why is sex determination so changeable during evolution while other developmental processes are strongly conserved? To answer this question information is needed about the underlying genetics as well as on the interaction between the sex determining mechanism and other aspects of the biology (life history) of a species. Our research aims at unraveling the genetics of haplodiploid reproduction and we use parasitic wasps as model organisms. In some haplodiploid species sex is determined by the allelic state at a single locus, but in others, like our model organism Nasonia, maternal effect imprinting is involved. We study which genes govern sex determination and the consequences of the sex determining mechanism for reproductive behavior and the survival of species.