The generation of the 2006 functionals was ingeniously crafted by merging the characteristics of the earlier M05 172 and VSXC 173 functionals (in turn designed from modifications of the PBE and LSDA functionals for the exchange). These include M06, a versatile hybrid meta-functional that boasts consistent accuracy for main group thermochemistry, barrier heights, medium-range correlation energies, and transition metals. M06-2X, another hybrid meta-GGA, excels in main group chemistry and barrier heights, accurately predicts valence and Rydberg electronic excitation energies, and π-π stacking interactions, while its performance falters in the realm of transition metals. M06-L, a local functional devoid of Hartree-Fock exchange, was skillfully tailored as a cost-effective choice for numerous demanding applications associated with extensive systems. It excels for transition metals, yet its accuracy for barrier heights does not match that of M06 and M06-2X. Finally, M06-HF was designed primarily for spectroscopy, demonstrating good performance for valence, Rydberg, and charge transfer excited states with little compromise on ground-state accuracy. An important point to note is that M06-2X and M06-HF that differ in the amount of exact exchange (54 vs 100 %) share the same training set, which was expanded with transition metals with respect to the one used for the parameterization of the M06 functional. revM06-L, on the other hand, was developed later using an even larger database and additional smoothness restraints to ensure better numerical stability, smoother potential energy curves, and overall improved accuracy compared to M06-L.