Mary Jo Duffy is an American comic book editor and writer, known for her work for Marvel Comics in the 1980s and DC Comics and Image Comics in the 1990s.

A comic book letter column is a section of an American comic book where readers' letters to the publisher appear. Comic book letter columns are also commonly referred to as letter columns, letter pages, letters of comment (LOCs), or simply letters to the editor. Letter columns appeared early on in the history of comic books themselves, and their growing prevalence — particularly beginning in the 1960s — helped create and legitimatize comics fandom. As the forum developed, the volume and tenor of letters became a reliable gauge of overall reader response to developments in the comics themselves. Letter columns remained a regular feature of most comic books until the early years of the 21st century, when they began being phased out in favor of the growing prevalence of email and Internet forums. Despite this, the 2010s saw a renaissance of comic book letter columns, and many comics titles still print them.