Abstract: The discovery of neutrino flavor oscillation, and the existence of non-zero neutrino masses, is the first observation of phenomena not allowed by the standard model of particle physics. In this colloquium I will describe the experimental evidence for neutrino oscillation and discuss the open questions of neutrino physics. I will focus on the hunt for the last neutrino mixing angle with the T2K experiment, which will send the world's most powerful accelerator neutrino beam 295 km across the island of Japan to the largest particle detector in the world.