In bulk metallic systems, electrons have the freedom to move throughout the 3-dimensional space. When the size of a given metallic system is reduced to nanometer scale along at least one direction, it is expected some electron confinement along that direction. These electronic states may be described as resonances arising from interface/surface reflectivity. Such confined states are likely to show effects not seen in the bulk structures and produce size dependent responses to external and other probes. These may be viewed as some sort of a resurrection of other-wise dead materials science problems. In a sandwich structure, where magnetic layers are separated by a nonmagnetic spacer, it is expected to see confined electronic states, sometimes referred to as quantum well (QW) states, when the spacer layer thickness satisfies appropriate boundary conditions. The chapter discusses the experimental evidence for the existence of confined states in multilayer systems and examines how far can simple theories of spin-dependent transport can be taken, provided k-resolution is available. Using high quality single crystal samples, it may be possible to obtain (and possibly tune) k-dependent transport. It is instructive to review some of the past work in this field, to understand the historical progression. The chapter discusses wedge shaped samples, phase accumulation model, interfacial roughness, envelope functions of the QW state, multiple quantum wells, non-free-electron-like behavior, reduction of the 3-dimensional Schrödinger equation, envelope functions, and the full problem. In principle, the relevant many particles Hamiltonian in all the different regions of the heterojunction carries all the necessary information and its appropriate eigenstates can be used to describe, for example, quantum tunneling in such a device. The chapter details the applications—confined states in metallic multilayers—spin transmission and rotations, angle resolved and inverse photoemission.