GPAW is an efficient, open-source (GPL) program package for electronic structure calculations based on the density functional theory and projector augmented wave (PAW) method. the program uses real-space formalism which allows flexible boundary conditions and efficient parallelization. Compared to more traditional plane wave or localized basis set approaches, real space grids offer several advantages, most notably good computational scalability and systematic convergence properties. However, as a unique feature GPAW also facilitates a localized atomic-orbital basis set in addition to the grid. The efficient atomic basis set is complementary to the more accurate grid, and the possibility to seamlessly switch between two representations provides great flexibility.