The e-cigarette company Juul, which won an investment from the tobacco giant Altria at a $38 billion valuation, faces an uncertain future. Analysts have credited Juul with single-handedly resurrecting the slumping e-cigarette market. But Juul is confronting challenges that include warnings from national regulators, investigations by members of Congress, state- and country-wide bans, and questions about its products' long-term health impacts. Juul's roots are in Silicon Valley, where it started as a tiny company called Ploom. Over time, Juul has attracted attention from an array of nontraditional investors, including a marijuana therapeutics company and several hedge funds.