Spider mites are increasingly serious pests of cotton and forage crops. Because the acaricides now being developed are often highly selective among species, entomologists concerned with the control of these pests need to know the particular species they are dealing with. This report has been prepared to aid them. Both field guides and taxonomic keys are given. Suggestions are offered on how to sample, how to prepare slides, and what diagnostic characters to look for. One new species from Nicaragua is described, and new synonymy is noted for several others. The species treated—all those reported from cotton in North and Central America—are: Petrobia latens (Muller) Paratetranychus peruvianas McGregor Paratetranychus mcgregori new species Tetranychus pacificus McGregor Tetranychus schoenei McGregor Tetranychus canadensis (McGregor) Tetranychus bimaculatus Harvey Tetranychus atlanticus McGregor Tetranychus marianae McGregor Tetranychus desertorum Banks Tetranychus tumidus Banks In addition, some species reported from cotton in other parts of the world are noted, and a new combination, Paratetranychus stenoperitrematus (Ugarov and Nikolskii), is given for one of them.