Mobile data usage is growing exponentially driven by smartphones, but most data usage occurs indoors where wireless coverage is poor. Traditional macro-cellular networks cannot meet long-term mobile broadband demands. Femtocells provide a solution by creating "personal" indoor base stations that improve coverage and capacity through spatial reuse. Key challenges include interference avoidance between macrocells and femtocells as well as developing applications enabled by femtocell awareness of mobile and home networks. Femtocells promise benefits for both consumers through improved in-home experience and operators through lower costs and new services.