The Microsoft Cognitive Toolkit (CNTK) is an open-source, commercial-grade distributed deep learning tool. It uses directed graphs to describe the computational steps of a neural network, supports common model types, and implements automatic differentiation and parallel computing. CNTK supports 64-bit Linux and Windows operating systems and can be used as a library for Python, C or C++ programs, or as an independent machine learning tool using its own model description language, BrainScript.