Body composition assessment provides a sharper picture of the human biological response to genetic and environmental influences than measures of body size and weight. Infant body composition is particularly important as a marker of fetal adaptation and developmental programming of subsequent health and disease, but until recently, the range of options for measuring infant body composition was relatively narrow. The purpose of this Toolkit: Methods in Human Biology review is to provide a comprehensive overview of methods of body composition methods currently used in infants 0 to 2 years of age, including anthropometric prediction equations, air displacement plethysmography (ADP), dual energy X-ray absorptiometry (DXA), bioelectrical impedance analysis (BIA), isotope dilution, and magnetic resonance imaging (MRI). Information on the reliability, validity, and accuracy of the methods is provided. Unique aspects of infant physiology and behavior create challenges for body composition assessment, but this review provides guidance on suitable testing approaches and environments that may aid researchers in this important area of investigation. Am. J. Hum. Biol. 26:291-304, 2014.