"The support classes, Engineer and Medic, are the backbones of the team's efforts at both defense and offense, with the Sentry's Area Denial, the Dispenser's healing and ammo supply, the Teleporter's ability to transport teammates quickly to the frontlines, the Medi Gun's mobile healing on the battlefield and the raw, game-changing power of the Ubercharges. However, both classes are fairly squishy and needs to be alive near the frontlines for a relatively long period of time to be at their maximum effectiveness. Thus, the rest of the team (including you, unless you're being a support class yourself) should help them out as much as possible.

General

• Spycheck! Both classes tend to be prime targets for Spies. Although Pyro is the best class for that, anyone can just shoot at suspicious-looking teammates to prevent a backstab from crippling your team's power.
  
• Extinguish them if they're on fire! Either airblast them as a Pyro, use a Sandvich, Buffalo Steak Sandvich, Dalokohs Bar, Jarate or Mad Milk on them, or let them get to a Medkit first. Afterburn can take out quite a sizeable chunk of their health, making them vulnerable assuming they didn't just die.

with a Medic

• Do try to have at least one power class (Heavy, Soldier, Demoman) to go with the Medic so that they have someone to pocket. And stay with them. A Medic can be effective when he has a pocket to protect him.
  
• If a Medic chooses to Uber you, try to take out nearby Sentries, crowds of enemies, or to hold the frontlines/ a chokepoint. If everything seems fine, check behind! Your Medic could be under an ambush and is trying to save himself. Get rid of the threat.
  
• Signal the Medic when you notice threats ahead (Snipers, Sentries, large groups of enemies). Use voice responses, text binds, text chats, or if urgent, just fall back and spam "E".
  
• During setup time, if you notice your Medic is alone, stay near him so that he can build Uber to use when the round starts. Having an Uber is generally a really great advantage.

with an Engineer

• During setup time, if you are one of those classes that doesn't need to prepare (Medic, Demoman, Engineer, and maybe Heavy if there is a prime location you need to get to), follow the Engineer and suicide when he picked a spot for his Sentry nest, so that he can use the metal to drop to setup.
  
• During setup time, if the Engineer sets up a Teleporter, you can briefly switch to Engineer and help upgrade it, lessening the Engineer's workload. Upgrading one end of the Teleporter automatically upgrades the other end when both ends are up, and since you're at spawn and near a Resupply Cabinet, it'll be easier fro you to do so than him. Just switch back to whatever class you're playing and take the Teleporter to the frontlines when you're done.
  
• If an Engineer is setting up nearby, DO NOT grab any nearby ammo packs, use the Dispenser instead. Unless there is no Dispenser nearby and you desperately need ammo.
  
• If an Engineer is setting up close to the frontlines, guard him and buy him some space to build.

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