Although solute carrier (SLC) and ATP-binding cassette (ABC) transporters are critical to the absorption, distribution, and elimination of many small-molecule drugs in children, how these transporters regulate pediatric drug handling remains unclear. For proper dosing and to diminish toxicity, we need a better understanding of how organ development and functional maturation, as well as developmental changes in systemic physiology, impact transporter-mediated drug handling at pediatric developmental stages from the preterm infant through adolescence.