This guide accompanies the release of version 6 of the Open Source Field Operation and Manipulation (OpenFOAM) C++ libraries. It provides a description of the basic operation of OpenFOAM, first through a set of tutorial exercises in chapter 2 and later by a more detailed description of the individual components that make up OpenFOAM. OpenFOAM is a framework for developing application executables that use packaged functionality contained within a collection of approximately 100 C+ libraries. OpenFOAM is shipped with approximately 250 pre-built applications that fall into two categories: solvers, that are each designed to solve a specific problem in fluid (or continuum) mechanics; and utilities, that are designed to perform tasks that involve data manipulation. The solvers in OpenFOAM cover a wide range of problems in fluid dynamics, as described in chapter 3.