Federal Law

Marijuana is currently classified as a Schedule 1 controlled substance, implying that it has a high potential for abuse, no officially accepted medicinal uses, and no safe level of use under medical supervision. Heroin, LSD, and Peyote are other Schedule 1 drugs; cociane and PCP are listed in Schedule 2, allowing doctors to prescribe them. Under Federal law, it is illegal to posses any amount of pot anywhere in the United States. Penalties for a first marijuana offense range from probation to the death penalty. Moreover, it is illegal to use the U.S. Postal Service or other interstate shippers for the advertisement, import, or export of such marijuana paraphernalia as roach clips, water pipes, and, in some instances, cigarette papers- a crime that can lead to imprisonment and fines of up to 100,000.

Best Source for the laws state by state.
Norml

Yet you do have to ask yourself why!