FHI-gap: a GW code based on the all-electron augmented plane wave method. The GWGW method has become the state-of-the-art approach for the first-principles description of the electronic quasi-particle band structure in crystalline solids. Most of the existing codes rely on pseudopotentials in which only valence electrons are treated explicitly. The pseudopotential method can be problematic for systems with localized dd- or ff-electrons, even for ground-state density-functional theory (DFT) calculations. The situation can become more severe in GWGW calculations, because pseudo-wavefunctions are used in the computation of the self-energy and the core–valence interaction is approximated at the DFT level. In this work, we present the package FHI-gap, an all-electron GWGW implementation based on the full-potential linearized augmented planewave plus local orbital (LAPW) method. The FHI-gap code can handle core, semicore, and valence states on the same footing, which allows for a correct treatment of core–valence interaction. Moreover, it does not rely on any pseudopotential or frozen-core approximation. It is, therefore, able to handle a wide range of materials, irrespective of their composition. Test calculations demonstrate the convergence behavior of the results with respect to various cut-off parameters. These include the size of the basis set that is used to expand the products of Kohn–Sham wavefunctions, the number of View the MathML sourcek points for the Brillouin zone integration, the number of frequency points for the integration over the imaginary axis, and the number of unoccupied states. At present, FHI-gap is linked to the WIEN2k code, and an implementation into the exciting code is in progress.