Ultrasound is a remarkably powerful and versatile modality for pediatric imaging, without requiring exposure to radiation or sedatives. By providing information on blood flow, Doppler sonography can reveal details about normal physiology and disease processes not discernable from gray-scale anatomic images alone. However, in routine practice in many institutions Doppler remains underutilized, in part due to uncertainty of the meaning of changes in Doppler waveforms. In part I of this review, the basics of hemodynamics and how changes in blood flow affect the Doppler waveform are reviewed. Clinical applications in the investigation of hepatic disease are then discussed.