libnabo is a fast K Nearest Neighbour library for low-dimensional spaces. It provides a clean, legacy-free, scalar-type–agnostic API thanks to C++ templates. Its current CPU implementation is strongly inspired by ANN, but with more compact data types. On the average, libnabo is 5% to 20% faster than ANN. libnabo depends on Eigen, a modern C++ matrix and linear-algebra library. libnabo works with either version 2 or 3 of Eigen. libnabo also depends on Boost, a C++ general library. libnabo was developed by Stéphane Magnenat as part of his work at ASL-ETH and is now maintained by Simon Lynen.