In this paper we look at the developmentof ScaLAPACK, a software library for dense and banded numerical linear algebra, and the NAG Numerical PVM Library, which includes software for dense and sparse linear algebra, quadrature, optimizationand random number generation. Both libraries are aimed at distributed memory machines, including networks of workstations. The paper concentrates on the underlying design and the testing of the libraries. Keywords BLAS, LAPACK, NAG, numerical libraries, numerical linear algebra, numerical software, ScaLAPACK