Self-consistent density functional theory (DFT) is used to study the structure and active sites in unpromoted and promoted MoS2-based hydrodesulfurization (HDS) catalysts. A model consisting of single-layer MoS2 chains with and without promoter atoms located at the edges is used to represent the structures in the catalysts, and full relaxation is allowed to find the lowest energy configurations. The results show that the most favored edge structures deviate significantly from those considered in the literature and those expected from simple terminations of the bulk MoS2 structures. The calculations also show that the promoter atoms prefer to be located at the so-called sulfur-terminated ((1) over bar 010) MoS2 edges. Although such structures have not been considered previously it is found that they are in agreement with available structural information from Extended X-Ray Absorption Fine Structure (EXAFS) experiments. Since the creation of sulfur vacancies is believed to be the first step for many hydrotreating reactions, the energy required to remove sulfur from the different structures has also been calculated. Comparison with catalytic activity results for MoS2, Co-Mo-S, Ni-Mo-S, and Fe-Mo-S structures shows that the highest HDS activity is obtained for the structures with the lowest metal sulfur binding energy, in general agreement with the bond energy model (BEM). A more detailed analysis of the sulfur bonding in promoted MoS2 structures based on a simple LCAO-type model explains the origin of the different promotional behaviors. Finally, the adsorption of hydrogen on the different structures is discussed. We find hydrogen adsorption at edge sulfur atoms to be strong, and suggest that the S-edge is partly covered by SH groups during catalysis,