What are the "driving forces" responsible for various physical properties of alloys? What causes alloy systems to order chemically and/or magnetically? To answer these questions, we have been using a quantum mechanical (QM) method to calculate cohesive energies, magnetic properties, and thermodynamic phase instability of alloys. This scheme directly incorporates the inherent disorder of the high-temperature solid solution and in this sense goes beyond traditional "band theory." Although approximations have been made at various steps in the thermodynamical averaging, and in the QM treatment of the electrons, adjustable parameters have not been used. Recently, for FCC NiFe (INVAR) alloys, we have calculated cohesive energies and magnetizations for several concentrations, including Ni-35%, and, also, the short-range ordering instabilities in the Ni-rich alloys to investigate the strong interplay between magnetism and concentration fluctuations. We describe the current picture of NiFe INVAR alloys that emerges from our calculations.