The advent and advancement of ultrasound technology have proved to be invaluable tools in the assessment of the fetus while still in the intrauterine environment. Ultrasound has given and continues to give us fresh insights into many aspects of the developing human. The area of fetal growth and development is a difficult one. Our lack of knowledge of the basic controlling influences has inhibited investigations in this area, but ultrasound has at least allowed us to begin to scratch the surface of this complex subject. We can now obtain very useful clinical information that usually has excellent correlation with gestational age, fetal size, fetal structure, and fetal function. We are just now beginning to document the natural history of many fetal abnormalities. Undoubtedly, advances in our knowledge of genetics and molecular biology and continuing technologic advances in imaging techniques will provide fresh insights and allow us to continue to probe and unlock the mysteries of fetal growth and development.