TARDIS' current look
|First Appearance:||The Pilot Episode (1964)|
|First Official Appearance:||The Unearthly Child (1964)|
The Chameleon Circuit is what allows the TARDIS' exterior shell to change it's look, acting as the ships 'cloaking device'. It allows the ship to change it's color, mass, and texture of the shell in order for it to blend into it's surroundings. This device what created to help prevent any history being changed, not only that, but the Chameleon Circuits Telepathic Circuits causes people walking by not to notice that fact that it may stand out thus causing people less likely to walk into or even notice the TARDIS' existence. This only works for people that are not actively looking for the TARDIS or know what it is. If it is abandoned it will change its appearance yet again to avoid attracting more attention as a result of it deactivating most of it's systems. The options open for what the TARDIS can look like is near enough infinate (Trees, Cars, Police Boxes, Caravan etc) the Iconic Blue Police Box is the TARDIS' current appearance due to the Chameleon Circuit breaking after changing into the Police Box.
Usually, it's the operator(s) or Driver(s) of the TARDIS that chooses what the look of the ship wil be upon arrival. It was mentioned that Type 58 TARDISes can scan the target area and automatically blend in to it's surroundings right before materialization and select a disguise that will hide it from view.