Birth weight is a biometric measure of well-being widely used as an infant health indicator. It also offers insights into maternal and population health more generally. The most common measures of weight at birth are the mean and the proportion of low birth weight (LBW; less than 2,500 g) infants. LBW neonates experience higher risk of infant morbidity and mortality. Globally, LBW rates average 15%. Wealthy Western societies generally experience the highest mean weights whereas the lowest are found in some of the globe’s poorest nations. Factors affecting newborn weight fall into five categories: genetic, environmental, gestational, socioeconomic, and nutritional. Studies of birth weight concerned with change over time reveal important regional and temporal differences, notably during times of social and economic crisis. Numerous studies have identified relationships between low birth weight and a range of health problems in later life, including hypertension, coronary heart disease and non-insulin-dependent diabetes.