How do I get medals / promotions in Alien Swarm?

I see them on people, and they can be low level or high level, and I don't know where they get them from.


After you hit the level 27 (which is the maximum), you get the option to take a promotion, which will reset you to level 1 and relock all the weapons, but give you a badge. At present, you can be promoted up to six times.

For more info, see Promotion on the Alien Swarm wiki.