Intrauterine growth restriction (IUGR) is a frequent cause of perinatal morbidity as well as of impaired growth during childhood. Therefore, a clearcut definition of IUGR to identify those babies at risk is essential: The label IUGR generally should be assigned only to those infants with birth weight and/or birth length below the 10th percentile for GA with a pathologic restriction of fetal growth. According to the recent literature, clinical classification of the retarded babies seems to be less significant. Among the etiologic factors responsible for IUGR, one-third of the variations in birth weight are determined by genetic variables, two-thirds by environmental factors. In spite of the fact that a long list of established, different etiologic factors is known, in at least 40% of children no underlying pathology can be identified. Among the preventable, environmental causes of IUGR, smoking of the mother during pregnancy is by far the most important one, which is responsible for more than one third of all IUGR newborns.