The Defense Force Field Prisms are created by the Force Field Generator (see illustration). When activated, these gravimetric shields surround a TARDIS and protect against almost any electro-magnetic radiation, physical, electrical discharge, sonic, magnetic, gravitic, psionic, and chronon forces. It can even deflect the blast from a starship's planetary bombardment missile. When they are active, a TARDIS can withstand the destruction of a planet and the surface of a quasar or a neutron star. Attempts to paint a TARDIS with active force-fields will fail, as the paint will simply fall off.

The Force-Field system can be slaved to be activated by the HADS (see the next entry). The Force-Field Generator is found within the Master Control Console, as is the Gravitic Anomaliser, which shields a TARDIS from gravitic forces. The Prisms can also be reprogrammed to serve as an energy field that will block sun light over a very large area, replicating the effects of a solar eclipse.

The Force-Fields do not prevent the use of a TARDIS's communications systems. The Defense Fields for the Main Doors are on a separate circuit, which normally must be switched off for the doors to be opened. Thus it is possible to enter or exit a TARDIS while all parts of the Exo-Shell (except the Doors) are protected. The main Force Field acts to filter the air if the Main Doors are open. With some difficulty, the field for the doors can be extended beyond a TARDIS's Exo-Plasmic Shell. This can be used to create a pressurized area or corridor with a range of about 10 meters.

Only beings such as the Guardians, Eternals and higher life forms can penetrate a TARDIS's Force-Field Prisms by rotating the frequency of their attacks. A beam of Coherent Tunneled Chronons is also capable of damaging a TARDIS, as is a black hole, Dalek Temporal Disruptors, a supernova, the D-Mat Gun, or a wormhole. Time displacement waves found near a black hole can penetrate the force fields and kill the crew (though the TARDIS will be undamaged).