BondageJaguar, when you enlist they usually conduct a background check, basically making sure you aren't a felon, or have a record, warrants, etc. The security clearance, either Secret or Top Secret is extensive. It takes hours to fill out all the information, if you have this done you know it. Your average enlistee will not have a security clearance packet sent through because they are expensive and time consuming...they average about a year to come back with an answer.
The last one I did was a real pain...I've moved a lot the last 7 years, since I've been in school, and had a lot of part time jobs. Well, you have to have a different person that knew you at each address, and every manager listed. You have to have every day accounted for, and cannot have any gaps.
Interestingly, the most common cause for a no-go on a security clearance is bad credit...