Mathematics accepts the Roman numerals as symbols for definite numerical values, without regard to the question of their origin and early history; but this paper is concerned primarily with the form and origin of these symbols, and only incidentally with their numerical values. Yet, though our subject is only indirectly related to mathematics, a survey of some of the hypotheses that have been put forward to account for the forms of these symbols cannot but prove interesting to mathematicians, as the symbols were in general use until the sixteenth century and are still current everywhere, on the dials of clocks, the title-pages of books, and in printed references to authorities.