Highest Occupied Molecular Orbital (HOMO, engl. ) Denotes the highest occupied molecular orbital of a molecule (see also molecular orbital theory).

In one molecule different molecular orbitals are available, which are occupied by the electrons present. These orbitals are different energetic. They shall be filled by increasing energy level. The HOMO is the highest-energy occupied orbital. Of importance to the energy difference between the HOMO and the LUMO is (Lowest Unoccupied Molecular Orbital ). From the amount of the energy difference depends on how easily reach the electron- excited state.

In organic solar cells is achieved by the combination of different materials with different energy differences between the HOMO and LUMO ( heterojunction ) that are excited by the incident light states ( excitons ) can be better split. ( A splitting of the excitons is needed to win free charge carriers and thus electricity from the solar cells. )

For bimolecular reactions it comes to the interaction between the HOMO and the LUMO of both reactants. This interaction is referred to as charge-transfer interactions.