We summarize the MOLGW code that implements density-functional theory and many-body perturbation theory in a Gaussian basis set. The code is dedicated to the calculation of the many-body self-energy within the GW approximation and the solution of the Bethe–Salpeter equation. These two types of calculations allow the user to evaluate physical quantities that can be compared to spectroscopic experiments. Quasiparticle energies, obtained through the calculation of the GW self-energy, can be compared to photoemission or transport experiments, and neutral excitation energies and oscillator strengths, obtained via solution of the Bethe–Salpeter equation, are measurable by optical absorption. The implementation choices outlined here have aimed at the accuracy and robustness of calculated quantities with respect to measurements. Furthermore, the algorithms implemented in MOLGW allow users to consider molecules or clusters containing up to 100 atoms with rather accurate basis sets, and to choose whether or not to apply the resolution-of-the-identity approximation. Finally, we demonstrate the parallelization efficacy of the MOLGW code over several hundreds of processors. Program title: MOLGW Catalogue identifier: AFAW_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AFAW_v1_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: GNU GPL v3 No. of lines in distributed program, including test data, etc.: 167871 No. of bytes in distributed program, including test data, etc.: 1309269 Distribution format: tar.gz Programming language: Fortran 2003 with a few C subroutines, Python scripts. Classification: 7.3, 16.6, 16.10. External routines: libint [2], libxc [3], SCALAPACK [4] (optional) Nature of problem: Prediction of the electronic structure of atoms, molecules, clusters with a particular interest in their spectroscopic features, such as quasiparticle energies and optical spectra. Solution method: Using the GW approximation to many-body perturbation theory, MOLGW calculates total-energies, quasiparticle energies, and optical excitations. Additional comments: Python3 is required to run the test suite provided. Running time: From 30 s to a few hours References: [1] http://www.gnu.org/copyleft/gpl.txt[2] https://github.com/evaleev/libint[3] http://www.tddft.org/programs/octopus/wiki/index.php/Libxc[4] http://www.netlib.org/scalapack