It has recently been postulated that viscous flow, or plastic deformation, occurs near the tip of an advancing crack in glassy amorphous polymers such as Plexiglas, and this results in molecular orientation at the fracture surface. The evidence for this has been the appearance of colors on fresh fracture surfaces of Plexiglas and the extremely high measured surface work which is 1000 times greater than the theoretical surface energy. The effect of crosslinking has been studied by measuring the surface work of crosslinked forms of Plexiglas and highly crosslinked thermosetting polymers. The surface work decreases as the degree of crosslinking in Plexiglas increases. Colors are absent on the fracture surface of the highly crosslinked Plexiglas, and the surface is mirror-smooth. The effect of crosslinking is to inhibit viscous flow or plastic deformation at the tip of the crack. The surface work of polyester and epoxy resins have been measured and found to be a factor of 10 smaller than the linear polymers, but still remain much higher than the theoretical estimates. Preorienting Plexiglas or polystyrene reduces the surface work of a crack propagating parallel to the orientation and increases the surface work of a crack normal to the orientation. The colors on the fracture surfaces of preoriented Plexiglas samples either disappear or change to colors of shorter wavelengths if the crack is propagated parallel to the orientation. This is a good indication that orientation occurs at the tip of a crack and is responsible for the appearance of colors on Plexiglas fracture surfaces. The surface work of 150% hotstretched polystyrene is 7000 erg/cm.2, and the fracture surface is mirror-smooth. This surface work is greater by only a factor of 10 than the theoretical estimate and suggests that only a small amount of molecular motion has taken place.