Ichneumonids are parasites on insect larvae or pupae, on spiders, and in spider egg cocoons. Host selection is determined partly by ecological factors and partly by physiological, the ecological factors being more important than usually realized. Size and sex ratio, but not color or morphology, are commonly influenced by the host selected. The number of broods per season is probably sometimes dependent on the number of broods of the host. Moisture, particularly in the form of dew, is an important requirement of ichneumonids, and in many cases its scarcity is a limiting factor. Biological control could be made more effective if these facts were better recognized.