Terrestrial invertebrates experience hypoxia in many habitats and under a variety of physiological conditions. Some groups (at least insects) are much more capable of recovery from anoxia than most vertebrates, but there is still a tremendous unexplained variation in hypoxia/anoxia tolerance among terrestrial invertebrates. Crustaceans and arachnids may be less often confronted with hypoxic environments than insects and myriapods and also seem to be less hypoxia/anoxia tolerant. Tracheated groups, especially those that are able to ventilate their tracheal system like many insects, cope with lower critical PO2 than nontracheated groups. Modulation of oxygen carrier proteins is normally not important in hypoxia resistance. Recent application of genetic and cellular tools are revealing that many of the same pathways documented for mammals (e.g. HIF, nitric oxide) function to regulate morphological and biochemical responses to hypoxia/anoxia in insects.