Although there are only two sexes, the genetic mechanisms that determine male or female development are bewilderingly diverse throughout the animal kingdom. This applies also to insects who determine their sex by a cascade of genes that ultimately set the switch into a male or female mode. This thesis presents novel insight into the sex determination cascade of four parasitoid wasp species. These species belong to a group of insects that do not have sex chromosomes. Instead, males develop from unfertilized haploid eggs, and females from fertilized diploid eggs. In the sequenced genomes of all four species the sex determination genes were identified and compared, showing that the four sex determination cascades have even more variation that originally assumed. Sex determination in fertilized eggs is initiated by an interaction between gene products that the mother wasps put into their eggs and gene products that are derived from the father’s genome in the embryo. However, in some species, females are infected with a bacterium that manipulates their reproduction. Such females produce only daughters from unfertilized eggs. To understand this phenomenon, the expression of the sex determination gene products were determined in infected and uninfected females and their embryos. This showed for the first time that the bacteria changes the maternal supply of gene products in such a way that their eggs do not require a paternal genome anymore for female development. Hence, the bacteria have taken control over the sex determination of their wasp host.