Laboratory and field studies on about 100 species in sixteen families indicate that several factors can influence offspring sex ratios (males: females) in parasitoid wasps. For many species, the offspring sex ratio increases with one or more of the following: (1) maternal age at ovipositing or number of days since insemination; (2) the age of the male parent or the number of times he has copulated; (3) extreme temperature; (4) decreasing host size, age, or quality; (5) female wasp density; and (6) the number of progeny per host. Other factors which have been shown to affect offspring sex ratios in some species include (1) number of hours since insemination; (2) genetic factors; (3) maternal size; (4) maternal die; (5) polyembryony; (6) photoperiod and relative humidity; (7) hos sex; and (8) host density. These factors may affect offspring sex ratios through females manipulating fertilization of their eggs or through other mechanisms, such as differential mortality or changes in sperm availability. Theoret...