Eigen is primarily MPL2 licensed. See COPYING.MPL2 and these links: http://www.mozilla.org/MPL/2.0/ http://www.mozilla.org/MPL/2.0/FAQ.html Some files contain third-party code under BSD or LGPL licenses, whence the other COPYING.* files here. All the LGPL code is either LGPL 2.1-only, or LGPL 2.1-or-later. For this reason, the COPYING.LGPL file contains the LGPL 2.1 text. If you want to guarantee that the Eigen code that you are #including is licensed under the MPL2 and possibly more permissive licenses (like BSD), #define this preprocessor symbol: EIGEN_MPL2_ONLY For example, with most compilers, you could add this to your project CXXFLAGS: -DEIGEN_MPL2_ONLY This will cause a compilation error to be generated if you #include any code that is LGPL licensed.