In 2016, Americans bought about $7 billion of legal marijuana, roughly five times the 2013 total. Though the plant remains illegal in the eyes of the federal government, it is now the basis of what is arguably the country’s fastest growing industry.

California voters legalized cannabis for medical use in 1996, and additional states followed, but for years federal laws were arbitrarily enforced in those places. The industry’s rapid growth didn’t begin until after August 2013, when then–deputy U.S. Attorney General James M. Cole wrote a memo guiding federal law enforcement not to focus on marijuana businesses that followed state laws—even in the handful of states that were legalizing it for recreational use.

After the Cole Memo, companies felt more secure investing in their future. As the threat of federal prosecution dissipated, more skilled professionals joined the green rush. Colorado’s first recreational shops opened in January 2014, and they have thrived under the Cole Memo’s protection.