The ability to understand and control the unique properties of interfaces has created an entirely new field of magnetism, with profound impact in technology and serving as the basis for a revolution in electronics. Our understanding of the physics of magnetic nanostructures has also advanced significantly. This rapid development has generated a need for a comprehensive treatment that can serve as an introduction to the field for those entering it from diverse fields, but which will also serve as a timely overview for those already working in this area. The four-volume work Ultra-Thin Magnetic Structures aims to fulfill this dual need. The original two volumes - now available once more - are An Introduction to the Electronic, Magnetic and Structural Properties (this volume) and "Measurement Techniques and Novel Magnetic Properties." Two new volumes, "Fundamentals of Nanomagnetism" and "Applications of Nanomagnetism, " extend and complete this comprehensive work by presenting the foundations of spintronics.