An "active shooter" is an individual or small group actively engaged in killing or attempting to kill people, often in a confined and populated area; in most cases, using firearms although explosive devices in conjunction with firearms are becoming used with increasing regularity. If unfortunate enough to be caught in an incident, think "Run. Hide. Fight."
Firstly, let's look at "Run." Although this sounds simple enough, you must think before you run blindly into trouble. Here are a few things you must consider. Know where you are going to run, and know that you are not running into the threat. When you come to a door, wait, listen, peek—do not run blindly into areas.
Whenever it's a location, such as a restaurant, always take a few extra seconds to identify the nearest escape route. Quite often it will be a fire exit. If an incident were then to occur, you can react immediately and run from the threat. Remembering to never run blindly, there could always be more than one threat. If running will put you in more danger, you can always kneel or crawl and use cover.
If you think you hear gunfire, react immediately. If you see a threat, react immediately and run if possible. Think of any way to create barriers between you and the threat. If a fire exit is not available, think about windows and roofs. Remembering to never jump blindly or in a panic, a fall could kill you.
Now, let's look at "Hide." If moving away from the threat is not feasible and it is safer to stay where you are, then there are a few things you must consider. Look for somewhere which will give you cover from view and cover from gunfire. If hiding in a room, lock the door and place as many objects up against the door as possible. Turn off any TVs or radios and change your phone to silent. Move away from doors and windows. The attackers may try and shoot through these. And find the best possible hiding location. Always look for every opportunity to escape.
Last is "Fight." This is the worst-case scenario. You cannot run or hide, so now you fight for survival. Become the most aggressive you can and use any weapons available. If in a group, all attack at once. Rush the target at varying angles and commit to taking them down. You now have no other option and will be fighting for your life.
Now, some key points to remember. As soon as you hear or see gunfire or what may be gunfire, react immediately. Get down and move away from that direction, under cover wherever possible—crawling if needs be. If you hear or see an explosion, react immediately and find cover. Get low; stay low. Assess the situation and move quickly away from danger. There is a strong chance that secondary attacks or explosions will follow. Do not get drawn into crowds or move into rescue mode. Immediately and quickly, move away from the immediate area and focus on survival.