Publisher Summary This chapter presents the classification of insect relationship on the present hypotheses. Within the class Insecta, major forms of insects are grouped in orders. Ordinal-level groups represent divergent lineages that are nearly always recognizable by a set of distinctive characteristics. Almost always, an adult insect can be readily determined to order at a glance. In many classification systems, orders are grouped into superorders but what comprises a superorder is far from fixed. For example, zorapterans are viewed as being in the superorder Orthopteroidea by some and in the superorder Hemipteroidea by others. Although a close link between Trichoptera (caddisflies) and Lepidoptera (moths and butterflies) is supported by a wealth of concordant evidence and consequently is undisputed, relationships among many endopterygote orders are unclear because different data sets present conflicting evidence. Some orders are considered to be noninsects (placed in the class Parainsecta), but evidence clearly places them in the superclass Hexapoda with insects.