A monoid hypersurface is an irreducible hypersurface of degree d which has a singular point of multiplicity d-1. Any monoid hypersurface admits a rational parameterization, hence is of potential interest in computer aided geometric design. We study properties of monoids in general and of monoid surfaces in particular. The main results include a description of the possible real forms of the singularities on a monoid surface other than the (d-1)-uple point. These results are applied to the classification of singularities on quartic monoid surfaces, complementing earlier work on the subject.

22 pages, 4 figures