EFANNA is a header-only C++ library for approximate nearest neighbor (ANN) search. It builds an index combining a k-nearest neighbors graph and tree structures (KD-trees or hash tables) to enable efficient search on high-dimensional data. The library provides optimized distance computations with SSE/AVX intrinsics and a matrix class for SIMD-friendly data access. Key functionalities include index building, saving/loading indices, and performing ANN search via command-line examples or a MATLAB interface. The architecture is modular, with core algorithms in base_index.hpp and specialized index implementations in kdtreeub_index.hpp and hashing_index.hpp.