I'm a 33 yr Teamster driver. Although I don't need a HAZMAT endorsement on my license for my primary job. I always had it for some side jobs I did. It's basically a written test to show understanding of all the harardous cargo u may haul. For example, flammables, explosives, chemicals etc. You have to show an understanding of what chemicals can and cannot be hauled together, u have to show an understanding of how to deal with said cargo in the event of an accident. You have to know how to placard your load. And much more but u get the picture. After 9/11 u had to undergo a background investigation of sorts. Seems like a good idea except it's expensive and most drivers have to pay out of pocket. It's also not an easy test to pass. I let my endorsement expire when it became to expensive.