I give a brief history of nearest neighbor (NN) methods, starting from Alhazen’s “Book of Optics” in the 11th century, and leading up to present time. Surprisingly, the most general nonasymptotic results on NN prediction only recently emerged in 2014 in the work of Chaudhuri and Dasgupta. Turning toward “user-friendly” theory with an eye toward practitioners, I mention recent guarantees (2013-2015) in the contemporary applications of time series forecasting, online collaborative filtering, and medical image segmentation — in all three cases, clustering structure enables successful NN prediction.