This chapter contains sections titled: Introduction: Importance of Active Transport Mechanisms for Uptake, Tissue Distribution and Elimination of Xenobiotics Structure of the Human MDR1 Gene P-glycoprotein Expression in Healthy Tissues Function of P-Glycoprotein Intestinal P-Glycoprotein P-Glycoprotein and the Blood–Brain Barrier P-Glycoprotein in Other Tissues Identification of MDR1 Mutations and Their Consequences for Function Racial Differences in Frequency of MDR1 Mutations MDR1 Mutations and the Potential Risk for Idiopathic or Spontaneous Diseases P-Glycoprotein and Ulcerative Colitis P-Glycoprotein and HIV Infection P-Glycoprotein and Renal Cell Carcinoma Aspects of P-Glycoprotein Regulation