The purpose of shielding is to suppress radiated interference. More than a sheet of metal is involved. For good shielding, three essential conditions must be met: filters for all penetrating wires, prevention of imperfections and appropriate calculations. The size of the shielding enclosure plays a significant role, having opposite effects for magnetic and for nonmagnetic materials at low frequencies. This chapter investigates this in more detail and explores how to cope with the detriments imparted by actual structures. It discusses a ticklish problem: Shape factors, frequency dependency over broad ranges, and nonlinearity interact complexly. One has to be cautious in selecting an appropriate simplifying model that yields a realistic, useful answer, not just a mathematical exercise. The time needed to penetrate a shield turns out to be a key criterion. For ferromagnetic materials, the details of the magnetization curve become secondary.