The Main Doors serve as the primary entrance/exit of a TARDIS's interior Micro-Universe. They are found only in the Primary or Secondary Control room. The connection between the interior and the Exo-Shell is achieved with differential layer-slippage to create the minimum of user involvement in the mathematical sub-structure. TARDISes are designed not to affect history. Because of this crewmembers normally can't alter history unless they leave the TARDIS.

When set properly the main doors can be voice activated and deactivated by saying "Doors." There is also a mechanical lever on the console that can be used to open the doors. They can be set to automatically close if no one is in the Control Room. If all power has failed, the Main Doors can be "opened" using a large hand crank inserted into the wall to the left of the doors. It is impossible to open the doors without first shutting off the force-fields that protect the main doors.

The atrium zone between the inner and outer doors is usually a black spatio-temporal void, but can be rendered in curved mirrors. The inner and outer doors can act as an airlock that functions in vacuum or even under water. Normally it possible to see the exterior environment through the open doors, however when in airlock mode there is a black spatio-temporal void between the inner doors and the exterior doors on the outer shell. This void prevents the exterior doors from being seen from inside even though there are less then 2 meters away.

The Main Doors' interior (and exterior) size can be altered using the architectural software. This is done with a localized mass inversion wave, which returns the doors to a raw energy state. With access to a subject's biodata, it is possible to create an exclusion field that will prevent that person from entering the TARDIS. An Emergency Door located in the Power Stacks Room leads out of the opposite side of the Exo-Shell from the Main Doors. This Emergency Door functions as a more traditional airlock.

Dematerializing while the Main Doors are still open can result in crewmembers being sucked into the Vortex and disintegrated by the Time Winds. If the Door were opened while a TARDIS was traveling in the Vortex an emergency system would initiate an Automatic Emergency Landing. But even if the TARDIS Force Fields kept the crew from being sucked out into the Vortex there is still a good chance that the space pressure would compress the dimensional stability of the interior, drastically shrinking the TARDIS and its crew. This dimensional compression would reverse itself when the TARDIS next dematerialized.