The London force occurs between all types of molecule, but is the only force existing between molecules having no permanent dipole moment such as monatomic molecules (the noble gases), homonuclear molecules (O2, N2, I2 etc.) or symmetrical polyatomic molecules (CO2, CCl4 etc.).
Although their dipole moment is zero on average, fluctuations in the electron density cause instant non-zero dipoles that induce dipoles in neighbouring molecules.
The London forces contribute to the Van der Waals forces.