The present article will highlight some recent experimental and theoretical studies of both unpromoted MoS2 and promoted Co-Mo-S and NiMo-S nanostructures. Particular emphasis will be given to discussion of our scanning tunnelling microscopy (STM), density functional theory (I)FT), and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) studies which have provided insight into the detailed atomic structure. In accordance with earlier theoretical studies, the experimental studies show that the Ni-Mo-S structures may in some instances differ from the Co-Mo-S analogues. In fact, the Co-Mo-S and Ni-Mo-S structures may be even more complex than previously anticipated, since completely new high index terminated structures have also been observed. New insight into the HDS mechanism has also been obtained and complete hydrogenation and hydrogenolysis pathways for thiophene hydrodesuffurization (HDS) have been calculated on the type of structures that prevail under reaction conditions. It is seen that important reaction steps may not involve vacancies, and special brim sites are seen to play an important role. Such studies have also provided insight into inhibition and support effects which play an important role in practical HDS. Recent STM studies have shown that MoS2 clusters below 2-3 nm may exhibit new structural and electronic properties, and a large variety of size-dependent structures have been identified. In view of the large structure sensitivity of hydrotreating reactions this is expected to give rise to large effects on the catalysis. (C) 2007 Published by Elsevier B.V.