As-terminated Si surfaces are model semiconductor interface systems with a wide range of technological applications. We have studied As-terminated Si (001) surfaces with scanning tunneling microscopy which reveals that it is possible to produce a well ordered surface. Some characteristic types of defects do occur on this surface, however, which are found to be of particular interest. These include antiphase domain boundaries, step edges, and long, atomically straight trenches running perpendicular to the dimer rows across the surface. The nature and origin of these defects are discussed.