This chapter reviews the literature on classifying magazine and journal periodicals and points out to new theorizing on the question, research needs and opportunities. It examines three broad bodies of literature that bear on questions of classifying magazine and journal periodicals. The first is the specific literature related to defining and categorizing the broad range of serials that are non-newspaper periodicals, more commonly referred to as magazines and journals. The second is the taxonomic literature—that is, the literature that focuses on systematic classification for identifying and developing taxonomies, typologies and, from the humanities, genres. And third is the social communication and interdisciplinary literature that includes scholarly inquiry related to categorizing all forms of magazine and journal periodicals. Denis McQuail’s summary definitions of theory for mass communication scholarship and operational theory provide critical links for moving through the quagmire that marks the state of magazine classification.