In this lecture we describe the total-energy momentum-space formalism that is widely used to describe solids, molecules, and nanostructures using periodic boundary conditions. First we describe how the electrons in atoms can be separated into “frozen” core electrons and valence electrons, with the latter being responsible for chemical bonds in molecules and solids. We then describe how pseudopotentials are combined with periodic arrangements of collections of atoms (supercells), and plane-wave basis sets are used to effectively describe the electronic and structural properties of solids, surfaces, crystal defects, and molecules.