As smartphones have emerged as the most widely deployed mobile computing platform, the scale of smartphone experimentation has lagged behind. New facilities enabling large-scale experiments are needed to ensure that research discoveries translate to the billions of smartphones in use today. To meet this challenge, we introduce PhoneLab, a 288-device smartphone testbed deployed at the University at Buffalo. PhoneLab provides access to smartphone users incentivized to participate in experiments while simplifying experiment data collection. The testbed will open for public experimentation in October, 2013, and continue to expand in 2014. To demonstrate the power of PhoneLab, we present three selected results from a usage characterization experiment run on 115 phones for 21 days. We use each result to motivate a future PhoneLab experiment, demonstrating how PhoneLab will enable mobile systems research.