The eusocial, or most social of the insects, are the bees, wasps, ants and termites. The bees, wasps and ants are in the Order Hymenoptera, while the termites are in the Order Isoptera. All of the ants and termites are eusocial, while both the bees and wasps have many species that live solitary lives. This brief chapter of the social insects focuses on eusociality, which is the highest evolutionary grade and is defined by the combination of three traits: some form of care of the young, an overlap of two or more generations in the same nest site or bivouac and the existence of a reproductive caste and a nonreproductive or “worker” caste. It will be useful at this point to review the broad features of classification of the several groups of eusocial insects, with some of the terminology most often encountered in the literature.