Infant mortality is the death of an infant before the infant's first birthday. The occurrence of infant mortality in a population can be described by the infant mortality rate (IMR), which is the number of deaths of infants under one year of age per 1,000 live births. Similarly, the child mortality rate, also known as the under-five mortality rate compares the death rate of children up to the age of five.

In 2013, the leading cause of infant mortality in the United States was birth defects. Other leading causes of infant mortality include birth asphyxia, pneumonia, congenital malformations, term birth complications such as abnormal presentation of the fetus umbilical cord prolapse, or prolonged labor, neonatal infection, diarrhea, malaria, measles, and malnutrition. One of the most common preventable causes of infant mortality is smoking during pregnancy. Lack of prenatal care, alcohol consumption during pregnancy, and drug use also cause complications which may result in infant mortality. Many situational factors lead to infant mortality, such as the pregnant woman's level of education, environmental conditions, political infrastructure, and level of medical support. Improving sanitation, access to clean drinking water, immunization against infectious diseases, and other public health measures can help reduce high rates of infant mortality.

In 1990, 8.8 million infants younger than 1-year-old died globally out of 12.6 million child deaths under the age of five. More than 60% of the deaths of children under-five are seen as being avoidable with low-cost measures such as continuous breastfeeding, vaccinations, and improved nutrition. The global mortality rate in 1950 was 22.5% which dropped to 4.5% in 2015. Over the same period, the infant mortality rate declined from 65 deaths per 1,000 live births to 29 deaths per 1,000. Globally, 5.4 million children died before their fifth birthday in 2017.

The child mortality rate, but not the infant mortality rate, was an indicator used to monitor progress towards the Fourth Goal of the Millennium Development Goals of the United Nations for the year 2015. A reduction in child mortality is now a target in the Sustainable Development Goals—Goal Number 3: Ensure healthy lives and promote well-being for all at all ages. Throughout the world, the infant mortality rate (IMR) fluctuates drastically, and according to Biotechnology and Health Sciences, education and life expectancy in the country is the leading indicators of IMR. This study was conducted across 135 countries over the course of 11 years, with the continent of Africa having the highest infant mortality rate of any region studied, with 68 deaths per 1,000 live births.