An extensive density functional theory analysis is reported of the crystal-chemical properties, electronic band structure, and optical (absorption and electron energy-loss spectra) and phonon properties of bulk and mono- and bilayer molybdenite, a transition metal dichalcogenide with important applications in semiconductor technology. This mineral is characterized by virtually infinite bidimensional layers of MoS2(with covalent Mo—S bonds) held together by weak dispersive forces, explaining the easy cleavage of the (001) surfaces.