A way of combining Atomic Orbitals to encompass the whole molecule. These linear combinations of the electron orbits of the atoms provide an approximation to the electron density of the whole molecule. Obviously, this allows you to make estimations of the stability of molecules as well as providing some information about charged and excited states.

Hydrogen          Hydrogen Molecular Orbitals
 Atomic
  Orbitals
        _
       /|\ 
      / | \            _____________ sigma*
        |E            /         /^\
        |L           /           |
        |E          /            |electron destabilisation
        |C         /             |
2xH  ___|T_______ / _ _ _ _ _ _ _ 
        |R        \              |
        |O         \             |electron stabilisation
        |N          \            |
        |            \          \ /
        |E            \_____________ sigma
        |N
        |E
        |R
        |G
        |Y

• HOMO
• LUMO
• Bonding Molecular Orbital
• Antibonding Molecular Orbital
• Node