The Atmospheric Boundary Layer (ABL) is the lowest portion of the Earth's atmosphere which is affected significantly by the properties of the Earth's (land or ocean) surface. The ABL may show a large daily variation in wind, temperature, and stability or turbulence. The ABL is typically 0.5-1 km deep, representing roughly 10 percent of the total mass of the atmosphere. The lowest 10 percent of the ABL, or 50-150 m elevation, is called the Atmospheric Surface Layer (ASL); this is the region we live in, and some of its characteristics are important for a variety of applications. This document provides the basic physics background for applications such as the spreading of tracer clouds in the atmosphere, forest fires, optical refraction near the surface (mirages), etc. It is becoming customary to use the term Atmospheric (vs. Planetary) Boundary Layer (and Surface Layer) to distinguish from the comparable Oceanic Layers. Note that the top of the ABL is normally placed at 0.5-1 km, which is appropriate for mid-latitude conditions. However, at some low-latitude locations, such as the Persian Gulf or central Texas, the top of the ABL frequently reaches 3-4 km in the daytime in summer.