The document discusses the rise of innovation districts in America as a new model for fostering innovation. Innovation districts are geographic areas where anchor institutions, companies, startups, and business accelerators cluster and connect. They are physically compact, transit-accessible, and offer mixed-use housing, offices, and retail. Examples mentioned include Cambridge-MIT, Philadelphia-University City, and St. Louis-CORTEX. The key components that make innovation districts successful are their economic, physical, and networking assets as well as the overall innovation ecosystem.