This chapter is primarily meant to provide a background and a broad overview of the building sector and the status of energy use in buildings. After a brief historic background, the importance of buildings, both in terms of their asset value and energy consumption, is contextualized by providing pertinent statistics and presenting future trends relevant to the U.S. economy. This is followed by a background on the various categories of end uses in different types of residences and commercial buildings. Next, we present the numerous technical and specialized professionals and companies involved in building design and operation that have evolved over the years. We then discuss some basic concepts on economics (such as payback time and risk). The two types of unit systems, the SI and IP units, and how to perform interconversion calculations are then treated in view of the (unfortunate but realistic) fact that the latter is still widely used in the United States. Finally, we argue on the importance of being able to perform orders of magnitude calculations prior to embarking on a full-fledged analysis. The art of doing such back-of-the-envelope calculations is an essential skill that a scientist or an engineer should acquire and is a cornerstone of intuition. Further, it can often provide a valuable check for the reliability of results from more complicated analyses.