Among diamond-like carbon (DLC) coatings, hydrogenated amorphous carbon (a-C:H) films are of special interest, since some of them may exhibit coefficient of friction in the millirange, known as "superlow friction". This paper will review some key requirements for achieving this specific friction regime. At first, the formation of a carbonaceous transfer film on the steel counterface seems necessary for friction reduction. Oxide layers apparently slow down this build-up, as a-C:H surface reacts preferentially with metallic iron. Secondly, the formation and evolution of the transfer film on the sliding counterface seems to control the frictional behavior. By performing tribological experiments under different controlled environment, it is possible to have some clues on the evolution of the transfer film and on its relation with the friction level observed. Phenomena of loss and restoration of superlow friction are for instance very helpful. Finally, all hydrogenated amorphous carbon films do not lead to superlow friction under vacuum. The role of hydrogen content is known to be critical. but it appears to be strongly dependent on deposition process. Some mechanical properties of the films can also be correlated with friction reduction, like viscoplasticity. All these results suggest that surface rheological properties are of paramount importance in the achievement of superlow friction with a-C:H films. (c) 2004 Elsevier B.V. All rights reserved.