A report is made on the taxonomy and certain ecological aspects of the 30 species of opilionids found in the Great Lakes Region (Illinois, Indiana, Michigan, Ohio, Wisconsin and Ontario). Species reported in the literature as well as those found in collections made by the author and by others in the Region are included in an illustrated key to the species. Two forms of Odiellus pictus are given subspecies rank: O. p. pictus and O. p. argentus. A brief synonymy is given for each species. Body and leg lengths, brief discussion of both troublesome and conspicuous identifying characteristics and comments on the typical habitat are included. Remarks are made on distinguishing characteristics of the Order and on influence of life history stage and condition of the animal on ease of identification. The faunal list contains 18 species of Leiobunum; several species of this genus appear to reach their northern limits in this area.