The double burden of malnutrition is the coexistence of overnutrition (overweight and obesity) alongside undernutrition (stunting and wasting), at all levels of the population—country, city, community, household, and individual. This four paper Series explores how this coexistence is affecting low-income and middle-income countries. Malnutrition in its many forms has previously been understood and approached as a separate public health issue, but the new emergent reality is that undernutrition and overnutrition are interconnected and, therefore, double-duty actions that simultaneously address more than one dimension must be implemented for policy solutions to be effective. In addition to policy recommendations, the Series includes a focus on both historical and biological contexts, and new economic analysis.