A description of the male genitalia of the Pentatomidae is given, followed by a comparative study of these structures as found in Canadian species of this family. The development of our knowledge of the subject is indicated and the more important papers reviewed in the light of the author's findings. Each of the genital structures is discussed and described in turn. The act of copulation and the workings of the genital parts are described. Descriptions of a number of Canadian species of Pentatomidae are given in detail. A section is devoted to an explanation of the general line of development of the genital structures within the family.