Abstract: Sex allocation theory describes how parents should bias investment in either sons or daughters when each gives a different fitness return. Over the past decades, social Hymenoptera have increasingly been used as models for testing the predictions of sex ratio theory. Their haplodiploid sex determining system gives mothers considerable control over the proportion of each offspring sex by selective fertilization of eggs. Moreover, the great diversity in life-history strategies and breeding systems has allowed detailed tests of quantitative predictions linking sex ratios to environmental or genetic factors at the colony and population levels. Nevertheless, the vast majority of theoretical and experimental treatments devoted to sex allocation in ants, bees and wasps have focused on sex ratio at emergence or adult stage, rather than primary sex ratio adjustment (the proportion of each sex at oviposition). Sex ratio at emergence may be adaptive, but it may also result from sex-specific differences in brood mortality during development, whereas primary sex ratio can directly measure the mother's adaptive response to environmental variations and / or socio-genetic conditions. Here, I review current knowledge on primary sex ratio control by queens in social Hymenoptera, especially ants. I present the most classical methods for primary sex ratio determination, and outline empirical studies showing a regulation of the relative number of male and female eggs laid by queens as a function of demographic, ecological and socio-genetic factors. Finally, I propose some directions for future research that should help to clarify the extent of primary sex ratio adjustment in response to environmental conditions, the type of informative cues used by queens to assess their environment, and possible genetic constraints on primary sex ratio adaptation.