Fetal growth restriction is one of the most challenging obstetric problems. Due to confusing terminology, there is an obvious lack of uniform diagnostic criteria. At present, most authors do not distinguish between the terms 'small for gestational age' (SGA) and 'intra-uterine growth restriction' (IUGR). These two clinical entities are not the same. The term 'SGA' should be used for an infant who has failed to achieve a weight threshold (usually defined as the 10th percentile). Conversely, an IUGR infant has, by definition, not reached his/her genetic growth potential due to an insult that has occurred in utero. An IUGR fetus may or may not be SGA but it always implies a pathological process. Clinicians tend to manage both conditions in the same way by delivering affected cases, often by elective Caesarean section and often prematurely. This strategy obviously represents an overtreatment. This chapter will discuss the various aetiologies of small fetuses and will provide the reader with diagnostic tools to distinguish between subtypes.