Chapters 1 and 2 introduced the concepts of what radon is, where it comes from and its effect on the human body. This chapter will introduce how radon enters indoor air and how it behaves once indoors, i.e. how radon responds to the variable conditions inside a building. This study is important because nearly one out of every 15 U.S. homes are projected to have radon concentrations above the Environmental Protection Agency (EPA) guideline of 4 picocuries per liter (pCi/L).