Ivan Donald Margary, FSA (1896–1976) was a British historian who, during his lifetime, became the leading authority on Roman roads in Great Britain. He wrote numerous works on Roman roads of which his most influential and complete was Roman Roads In Britain. Margary's primary gift to the study of Roman roads was the development of a catalogue system known as Margary numbers, numbering Roman roads so that they could be referred to by catalogue number to avoid confusion, and to allow cross-referencing of the same road between different studies and authors.