Modern material design involves a close collaboration between experimental and computational materials scientists. To be useful, the theory must be able to accurately predict the stability and properties of new materials, describe the physics of the experiments, and be applicable to new and complex structures—the all-electron full-potential linearized augmented plane wave (FLAPW) is one such method that provides the requisite level of numerical accuracy, albeit at the cost of complexity. Technical aspects and modifications related to the choice of basis functions (energy parameters, core–valence orthogonality, extended local orbitals) that affect the applicability and accuracy of the method are described, as well as an approach for obtaining k-independent matrix elements. The inclusion of external electric fields is illustrated by results for the induced densities at the surfaces of both magnetic and non-magnetic metals, and the relationship to image planes and to nonlinear effects such as second harmonic generation. The magnetic coupling of core hole excitations in Fe, the calculation of intrinsic defect formation energies, the concentration-dependent chemical potentials, entropic contributions, and the relative phase stability of Zr-rich Zr–Al alloys are also discussed.