Disguise

From TF2 Wiki

The Spy is able to disguise as any character class of either team using his disguise kit. When the Spy disguises, he will be surrounded briefly by smoke and the glow of his own team color. Once disguised, the Spy will appear to the enemy team as the selected class and team color while wielding that class's primary weapon. To teammates, he will appear to be wearing a paper mask of the selected class, and his health indicator will note the team and class of his disguise. On console versions, spies take the host's name more often than not if disguised as the host's class or as a class not present on the enemy team: making the spy much more obvious to players on the host's team.

If a Spy disguises as a member of the enemy team, he will appear to enemies to have the name of a player on that team of the same class. If there are no players of that class on the team, he will have the name of another random player on that team. While disguised as an enemy, a Spy is able to receive healing from enemy dispensers, Carts and Medics. It is even possible to receive an ÜberCharge from an enemy Medic. Your disguised health level (not visible to you, only to enemies) will be randomly chosen to appear between 50%-100%. You can increase this disguised health level by being healed by a friendly or enemy Medic. Note that carts, dispensers and medkits only increase this fake health level if your real health is injured.

Note that enemy dispensers always try to heal you while disguised. This happens while cloaked! A smart Engineer (or any player) can see the team-colored healing line going from the dispenser to the cloaked spy, giving away the Spy's location. As a spy, you may want to sometimes change your disguise to your own team to avoid having enemy dispensers reveal you when cloaked.

Disguised Spies are unable to pass through enemy players or taunt, but can still use voice commands: to the Spy himself and to the enemy team, these commands will sound like the disguised class; to teammates, the Spy will sound normal.

While disguised, the Spy will slow down to match the movement speed of his disguised class (for example: disguised as Heavy, Soldier or Demoman). This speed reduction does not apply while cloaked. If the class is faster than the Spy (for example: disguised as Scout or Medic), he will move at default Spy speed.

A disguise is automatically lost if a Spy fires his revolver, swings his Knife or grabs the enemy Intelligence. Placing sappers or reloading your revolver will not remove the Spy's disguise.

Reloading the revolver looks as though the person you're disguised as is also reloading. However, this only works for the classes that can reload their primary weapon, so this will not work for Pyro, Sniper and Heavy disguises. You can only fake reload as long as your Revolver has less than 6 bullets in its clip, but because of this, once you have reloaded you will not be able to again unless you fire a shot (which will remove your disguise). You can avoid this by changing weapons before the reload animation is complete, though interrupting the reload makes the animation look broken (reloading while running makes it less noticeable). Watch the Spy Property Demonstration by [ESG]Orca for a demonstration of how it is done.

Disguising while cloaked will not display the smoke effect to the enemy team.

The spy has a keybind to don his last used disguise (default B)

Class Disguise	Effectiveness	Info
Scout	Low	A situational disguise. While the Scout has the smallest frame (making this disguise good for hiding) and is not suspicious while being around control points and Payload carts, the speed difference between a Scout and a disguised Spy is very noticeable, this is why it's best to run towards running players so they do not notice your decreased movement speed (running side by side will be a dead giveaway). Combined with the lack of double-jump this renders the Scout disguise more easily noticed by experienced players. Primary weapon carried is trivial.
Soldier	Medium	A Soldier disguise is useful in certain situations. Soldiers are common, so you may draw less attention from spy-checkers and do not look suspicious with their primary weapon equipped, but may look suspicious if you are not constantly firing it like other soldiers. There is also some chance an enemy medic may ÜberCharge them. The movement speed reduction makes backstabs more difficult, as well as making you more vulnerable to spy-checking (especially players running through you).
Pyro	High	One of the most useful disguises, although sometimes overused. Spies disguised as Pyros move at regular speed and do not look suspicious with their primary weapon equipped. They have an excuse to be hiding in corners, running around enemy lines and standing near sentry guns. Since any Spy should be staying far away from enemy Pyros, enemy players are also less likely to notice any name discrepancies. They are also a potential candidate for an enemy Medic's ÜberCharge. However set on fire while disguised as a Pyro, is a dead giveaway (real Pyros have flame retardant suits).
Demoman	High	Demomen often hang around enemy lines which makes this one of the best disguises, may also receive an enemy Medic's ÜberCharge. The Demoman primary weapon is often carried, and looks like the secondary weapon regardless. However, a Demoman who is not watching sticky bombs while inside the base, is suspicious. Demomen move with a negligible decrease in movement speed. (may make backstabbing slightly harder, only when trying to catch up to a moving opponent)
Heavy	Low	Rarely useful. Heavies have little excuse to be hanging around anywhere except the front lines and will be suspicious when not spinning the minigun once they get there. The significant decrease in movement speed also makes backstabs as difficult as possible, and the spy-checking is trivial. However, the Heavy is one of the best disguises for stealing an ÜberCharge from a foolish enemy Medic. A neat thing about this disguise is the fact that you appear to have a bigger hitbox than you really do, causing people aiming for your head to miss completely.
Engineer	Medium-High	The most useful disguise for disrupting enemy sentry guns. Engineers move at regular speed and generally run around the enemy base and near sentries. To their disadvantage, running around with the shotgun visible and not being seen building may raise suspicion, especially if there is equipment which is not fully built up. There is also the chance that you may come into contact with the Engineer that shares your name while attempting to sap their sentries. This disguise is possibly the best for disrupting sentries guarded by multiple Engineers, as it makes the Spy a less conspicuous target if he starts wreaking havoc with his sappers, since enemies will not know which of the several engineers is the spy. Unfortunately, this is a very common disguise and Engineers are commonly spy-checked.
Medic	Low	"Can you fix him, doctor?" A situational disguise. Enemies often have their backs to Medics, and are not surprised if you run behind them. Spies disguised as Medics move slightly slower than a real Medic's speed, but that will not usually give you away. However, wielding the syringe gun and not healing nearby teammates (which a Medic should be doing near constantly) instantly arouses suspicion from observant enemy players. Wielding a syringe gun can be suspicious since Spy disguises do not wield unlockable weapons, most medics carry a Blutsauger. Your biggest giveaway though is the lack of an ÜberCharge meter.
Sniper	Medium-High	Another of the more useful disguises. Spies disguised as Snipers move at regular speed, are expected to have the sniper rifle equipped and are expected to remain behind enemy lines, though they may draw suspicion running around the enemy base rather than remaining in a sniper spot and being unable to raise the scope to their eye. The Sniper disguise is useful for Spies making their way to enemy Sniper spots with less chance of being spy-checked, as well as sneaking up on the Snipers themselves.
Spy	Low	Another situational disguise. While Spies disguised as Spies will look suspicious running around the enemy base while appearing to be undisguised, the primary weapon carried is not suspicious (spies without a disguise, often carry a revolver). However, it fools enemies at range and gives an excuse to be running from the enemy calling for a Medic or to be mixed with opposing players. Unfortunately, many players instinctively shoot any spy on sight, including friendly ones. Spies disguised as Spies move at regular speed (note that even while disguised as an enemy spy, when you cloak it still glows your team color, and the enemies will not be able to see your shimmer while cloaked, so will know you're not their ally; because of this avoid cloaking around enemy players).
Any Friendly Class	Medium	Useful in a lot of situations. During setup time, Spies disguised as other classes on their own team can get an idea of the class distribution on the opposing side without alerting the enemy team to the fact that there is a Spy, while at the same time misleading the enemy. Likewise, it can be used to infiltrate the enemy base and be spotted without alerting the enemy to the presence of a Spy and giving them false info about the team's composition while saving charge on the cloak. Disguising as a stronger class may also allow fear tactics, for example: disguising as a Pyro to scare off enemy Spies. Spies may sometimes be able to draw enemy fire away from their teammates by disguising as a more important class, such as a Medic or Heavy. Disguising as a vulnerable class can fool the enemy into thinking they have the advantage, like a Sniper, as they run after what they think is a sniper, the Spy can cloak behind a corner, as they run past uncloak and backstab them. This tactic is also employed in Arena, when multiple people remain on a spy's team and only one remains on the other.

Additional Articles

See Spy strategy for information on how to infiltrate enemy territory effectively using disguises.

See Classes for details on the speed differences between classes.