How is medical marijuana even legal?

How do some states make medical marijuana legal, if U.S. Law says that *any* sort of possession of it is a criminal offense?

I've been reading statutes from different states that mention that it's not criminal to possess marijuana under a physicians prescription for a terminally ill, or otherwise ill patient.

Can't the FBI/DEA still bust you under U.S. Law?

: http://alcoholdrugstreatment.info/how-is-medical-marijuana-even-legal