Many deep learning and classical machine learning frameworks are useful but difficult to modify. In contrast, Flashlight, a C++ library for ML recently OSS’ed by Facebook, was designed to be customizable so that ML engineers and other developers can alter the low-level code to better leverage hardware and improve performance. Users can quickly and easily change its core components and use simple bridges to integrate code from low-level domain-specific languages and libraries. Flashlight includes apps for research across automatic speech recognition, image classification, object detection, and language modeling.