Q clearance will give you the ability to work on DOE/DOD Top Secret materials. If I recall correctly, there are three levels, P, L, and Q, with Q being the hardest to get. You pretty much have to have a spotless record and be a US Citizen or a Commonwealth citizen (i.e. U.K., Canada, Australia, etc.). Getting a 'Q' is time consuming and costs employers several thousand dollars. You don't get these things on your own like you would get a tech certification. I got my clearance in '99 while I was employed at Lawrence Livermore and it took a year from start to finish. Along the way I had my acquaintances interviewed by MIB types and I had a couple of taped interviews with the US State Dept. along the lines of 'Are you or have you ever been a member of the Communist Party...'.