HSL (formerly the Harwell Subroutine Library) is a collection of threadsafe ISO Fortran codes for large scale scientific computation, written by members of the Numerical Analysis Group and other experts. HSL was started in 1963 and over this long period it has reached a high standard of reliability and has a worldwide reputation as a prime source of numerical codes. Among its best know codes are those for the solution of sparse linear systems of equations, optimization, and sparse eigenvalue problems.

The latest version, HSL 2007, was released in October 2007. It includes a number of important new packages; these are highlighted within the Contents of the Library.