Amorphous coatings based on MoSx and hydrogenated carbon (a-C:H) exhibit dry sliding friction and wear behavior that qualifies them as ideal solid lubricating coatings. Both exhibit friction coefficients of 0.05 or less in dry air; in load-dependent friction tests, both coatings show interfacial shear strengths around 25 MPa in dry air. Both can have lifetimes of the orders of millions of cycles even though the coatings are one micrometer or less thick. However, their tribological behavior could not have been predicted based on common understanding of solid lubricants. The lubricity of MoS@sub 2@ is associated with easy shear of the basal planes, yet the MoSx-based coatings are amorphous. Often called diamond like carbon, a-C:H is a very hard material (12 - 16 GPa), with no easy shear planes. These discrepancies - between materials properties and tribological properties - can be reconciled by recognizing the role that third bodies play in the friction and wear of the two coatings. The low friction is associated with a film that transfers from the coating to the (counterface) slider and the velocity accommodation mode - interfacial sliding - between the transfer film and the parent coating. The higher friction coefficients of both in humid air is also explained by interfacial sliding and attributable to a higher interfacial shear strength in the presence of humidity. Earlier studies demonstrated that third body recycling from reservoirs contributed to the long life of MoS@sub 2@ coatings, and recent in situ tribometry studies also suggest ways in which the transfer film contributes to long life of diamond-like carbon, as well. Thus, both the friction behavior and the long life of these two otherwise different coating materials can be accounted for by third body effects on sliding contacts.