Most hymenopterous parasitoids are able to discriminate previously parasitized and healthy hosts, avoiding super- and/or multiparasitism to minimize intra- and/or interspecific competition for food. Chemicals usually mediate the discrimination. These chemicals are defined as host-marking pheromones (HMPs). The HMPs are deposited by ovipositing females on the host surface, on the ‘containers’ in which the hosts are concealed, in the patch where hosts are harbored, or/and are injected into the host body before, during or after oviposition. Source structures of HMPs are associated with Defour’s glands, poison glands, oviducts, ovaries, eggs or corpora allata. HMPs are detected with antenna or ovipositors. Up to now, several HMPs have been chemically identified. Most of them are blends of saturated and unsaturated hydrocarbons. For Dendrocerus carpenteri, however, it is juvenile hormone. Several factors, such as HMP persistence, parasitoid intrinsic (egg load, age, experience or learning) and extrinsic factors (parasitoid density, available hosts, or host species), may affect the response of parasitoids to HMPs. The theoretical importance and potential applications of HMPs are also discussed.