Marijuana remains a Schedule I controlled substance under the federal Controlled Substances Act even in the states that have licensed and taxed it, and that single unresolved conflict shapes every legal question a cannabis business, its landlord, its investors, and its lawyers will ever face. This guide explains the Controlled Substances Act and the reach of federal power confirmed in Gonzales v. Raich, the unstable history of federal enforcement discretion, and what rescheduling would and would not change. It then works through the concrete downstream consequences: why banking is scarce under the Bank Secrecy Act and FinCEN's 2014 guidance, why Internal Revenue Code Section 280E denies ordinary business deductions and can push a profitable dispensary into a loss, why federal bankruptcy relief is generally unavailable, why the Lanham Act's lawful-use requirement bars federal trademark registration, and how contract, insurance, employment, real estate, and interstate-commerce law all bend around the conflict. It closes with the separate and rapidly changing legal status of hemp and CBD after the 2018 Farm Bill and with the professional-responsibility question every lawyer in this field must answer before taking the engagement. The law described here is in motion at both the federal and state levels; verify current federal status and your own state's rules before acting.