Recent fights for a $15-an-hour minimum wage at Walmart and in the fast-food industry have interested academics, captivated the press, and energized the public. For good reason. The campaigns upend conventional wisdom about what unions do (help workers win collective bargaining rights) and why they do it (build the membership). Scattered flash strikes for seemingly impossible or idiosyncratic goals on no obvious timeline have shattered that mold. Though much has already been said about these developments, scholarship has yet to provide a rigorous theoretical frame to categorize and explain the new form of activism. This Article argues that improvisation—long the engine of comedy and jazz but more recently a topic of serious academic inquiry—does both. Improvisational unionism is an intentional social practice that galvanizes courageous conduct, inspires new relationships, and, most importantly, spreads. It also functions as a legal strategy selected for its unique potential to unlock worker militancy amid law and institutional restrictions that have corroded labor’s power for decades.