Nannochoristidae (=Nannomecoptera) and Boreidae (=Neomecoptera) are traditionally assigned to the holometabolan Order Mecoptera. Both are presently in the focus of insect systematists. They differ strikingly from the "typical" mecopteran pattern, not only in their larval and adult morphology, but also in their life style and reproductive biology. Phylogenetic affinities between Boreidae ("snow fleas") and fleas were postulated mainly based on molecular data, and morphological characters related to food uptake and larval features suggest a close relationship between Nannochoristidae and true flies. Both groups display fascinating features such as preferred temperatures around 0°C, extremely slender larvae adapted to running water, and a highly unusual mating behaviour. Despite of the extremely low number of known species, there is no doubt that both groups are key taxa and in their own way highlights in the evolution of the extremely successful Holometabola. This and an impressive number of recent studies on various aspects of Nannochoristidae and Boreidae more than justifies a new volume in the series Handbook of Zoology dedicated to these highly unusual taxa.