To interpret the results of hair analysis tests accurately and to understand the appropriate role of hair analysis in drug abuse testing, a basic understanding of the biology of hair is necessary. Although hair may appear to be a simple structure, it is actually a complex part of the anatomy whose biology is only partially understood. Hair grows from small organs (follicles) located within the complex microenvironment of the skin which has multiple layers of tissue, three glands whose secretions bathe hair, and multiple vascular systems which are capable of transferring drugs to hair at many levels along the length of the hair shaft. The advantages and disadvantages of using scalp, beard, or pubic hair as specimens for hair analysis are also considered.