How can I productively react when students misbehave or act up?

This is a tricky question. There are many thinks to take into consideration when you are dealing with a student, or students, who misbehave or act up. Is this there first time or does this happen often? What is the severity of their acting out? What is the whole situation at hand, were there others involved? The first thing you should probably do is find out what happened and, if you can, why. You need to analyze the situation to see if you can take care of it yourself or if you need to get somebody else involved. It also depends on the student (do they have emotional or mental disabilities? etc.). My biggest concern is knowing what, as a teacher, you can and cannot do as far as dealing with situation while it happens and also punishment. Obviously if it's something like acting out in class it's mostly up to your discretion, but what if it's something bigger such as a fight in the hallway?