Ambipolar diffusion is an important process in the initial stage of star formation whereby clumps of a molecular cloud uncouple from the interstellar magnetic field, which would otherwise resist the further gravitational collapse of the clumps. Magnetic fields thread all interstellar space and act upon ions (mostly protons) and electrons. These charged particles, in turn, collide with neutral atoms and thereby, in most cases, couple them to the field. However, in a molecular cloud, where the fractional ionization is very low (one part per million or less), neutral particles only rarely encounter charged particles, and so are not well-coupled. Thus, the interstellar magnetic field can drag the ions through the neutral gas without acting as a significant brake on the cloud's collapse.