This key provides an overview of Calanoida species known from North America, north of Mexico. This work is not intendent to be a monograph with detailed and exhaustive description of species morphology. Such information has been provided by monographs and papers listed in the reference section for each species. The species description is limited to the few key characters, which allow for proper species identification. In the following chapter, keys are arranged into plates with easy–to–use pictorial guide. Each key provides all relevant characters (the written description and drawings) important for family, genus, and species identification, which include distinction from similar group or species. The identification should always start from the beginning of the key. Otherways, a single character missed during the identification may lead to a wrong result. The species ecology chapter has all species to be organized in the alphabetical order, which is very practical and allows for faster search for desired information. Described also are recent changes (up to the December of 2025) in the taxonomy status of species and higher taxonomy levels up to the Calanoida order. Also, fixed are several errors, which several authors made in the previously published keys. The key presents 120 species (and one subspecies) reported from North America, which is 12 species more than was provided by the most recent key to the North American calanoid species), all illustrated with 121 maps, and 769 b/w illustrations, based on 1,220 cited primary reference papers. Provided also is the cumulative list of North American Calanoida species, with comments, linked with references to several older published keys.