PYRAMID is a software library used for the automatic adaptive refinement of large unstructured meshes on parallel computers and clusters. This software library can be easily used in a variety of unstructured parallel computational applications, including parallel finite element, parallel finite volume, parallel visualization applications using triangular or tetrahedral meshes, and quality controlled parallel mesh generation from initial coarse element models. The library contains a suite of well designed, and efficiently implemented, modules that perform operations in a typical parallel AMR process.