In the heat of the moment it can be hard to assess a dangerous situation and figure out the best way to respond. That's why police departments, colleges and the Department of Homeland Security are issuing guidelines for review in case of an emergency.
First, run. The Department of Homeland Security says if you hear shots fired or know of an active killing situation occurring, like the one in Ohio, but you do not see the suspect, then run. Get as far away as you can, and always have an escape route ready for places you frequent.
Also, leave your belongings behind, keep your hands visible and follow instructions issued by police or administrators.
They also say to not attempt to move people who have been injured.
If you can't run, or do see the suspect, the next step is to hide. Try to get out of the suspect's line of vision, and try and hide behind something that would offer protection from possible gun shots.
It's important to not trap yourself or restrict a possible escape route, however, police do suggest if you are in a room in a building to lock the door and blockade it with heavy furniture.
Always attempt to silence your cell phone and any other electronics so that you're not easily trackable.
If you are safe and able to, then call 911 to alert police to the suspect's location. If you can't talk because it would be dangerous, still call 911 and let the operator listen.
Last up is fight. That's of course an absolute last resort, and is only an option if your life depends on it. The best ways to fight back are by disrupting or incapacitating the suspect, which can be done by acting aggressively, throwing things or using nearby objects as weapons.
Police say it's important, however, to commit to your action: if you start to fight back, you should follow through.
You can read more from the Department of Homeland Security here.