Metallicity is the property of a metal to conduct electricity or heat. A simple metal is directly tied to its transport properties. However, transport depends on the type of material under observation. Electronic transport may depend on many factors such as the electronic structure, lattice vibrations (phonons), defect structure and related scattering mechanisms. In this chapter, special attention is paid to electronic transport that depends on magnetic interactions. Quantum mechanical effects, such as exchange (spin–spin) interactions, form an essential part of understanding of metals. Normal metals can be understood in terms of an effective one- particle theory. Electrons occupy one-particle (or quasi one-particle) levels when in their ground state. In metals at low temperature, there is a sharp cut-off in occupancy at the Fermi surface. This sharp cut-off is almost always responsible for various oscillatory features seen in metals. Such Fermi surface effects can be quite accurately calculated and understood nowadays using first principles band structure. For understanding the giant magnetoresistance effect and many other phenomena tied to metallicity, band theory has played a role and for that reason, there is review some of the modern band structure methods in the chapter. There is also some discussion about first principles band structure— basis sets for thin metallic films, full potential, some aspects of group theory, simple examples, and spectral representation. There is discussion on density functional theory: reduction of the many-electron problem, Itinerant magnetism, localized versus itinerant magnetic moments, Stoner criteria, Ruderman–Kittel–Kasuya–Yoshida (RKKY) theory and interlayer coupling, local spin-density functional, helical magnetic configurations: non-collinear magnetism, orbital and multiplet effects and current density functional theory. When the bulk crystal symmetry is broken, such as at an interface or a surface, enhanced orbital moments can be separated from spin moments and detected using experimental techniques such as magnetic X-ray circular dichroism.