During foraging and oviposition, hymenopteran parasitoid females often compete for hosts with individuals of the same or different species. The competition involves four types of behaviors: host-marking, ovicide or infanticide, brood-guarding and host-feeding. Advances in research of these behaviors are presented in detail in this review. (1) Host-marking has been documented in about 200 hymenopteran parasitoid species in nearly every super-family. Wasps mark exploited hosts physically or chemically before, during or after oviposition. However, most parasitoids utilize chemical markers, which are defined as host-marking pheromones (HMPs) or oviposition-deterring pheromones. HMPs usually mediate the discrimination between parasitized and healthy hosts, reduce super- and multiparasitism, and minimize intra- and inter-specific competition. HMPs can often decrease the tendency for a wasp to lay eggs in a marked host and promote dispersal. When HMPs do not completely suppress oviposition, they can reduce clutch size. Moreover, a gregarious wasp may modify the sex ratio of deposited brood in response to the presence or absence of an HMP. (2) Ovicide or infanticide refers to a parasitoid destroying an existing clutch of eggs or larvae in a parasitized host before laying its own clutch. A wasp usually commits ovicide or infanticide either by piercing eggs or larvae with its ovipositor, eating them, or injecting a toxic substance to the first brood(s) before or during oviposition. Generally, an adult kills brood(s) on conspecifically parasitized hosts more frequently than on hosts parasitized by itself. Ovicide and infanticide are advantageous since they remove the competitor(s) to restore, at least partially, the quality of parasitized hosts. (3) Brood-guarding behaviors are observed in many species in the family Bethylidae, Scelionidae, Pteromalidae, Mymaridae and Braconidae. Guarding wasps attempt to repel intra- and inter-specific intruders to protect their broods or unexploited hosts. (4) Adults of some parasitoids feed on host insects to obtain energy to develop eggs, a behavior defined as host feeding. A host-feeding wasp consumes healthy hosts or even directly kills the parasitoid broods in previously parasitized hosts. Females tend to feed on hosts when they possess lower egg loads than higher egg loads. The theoretical importance and potential applications of host competition during foraging and oviposition among hymenopteran parasitoids are also discussed.