Failure to thrive (FTT) is a syndrome of growth failure due to undernutrition. Determining whether an infant has FTT is based on the use of an anthropometric indicator and a selected cutoff value for that indicator. These anthropometric indicators include weight for age, weight for length, and length for age, and the cutoff values include the 10th, 5th, and 3rd percentiles. Each indicator and selected cutoff value provide unique information about an infant's growth. However, these parameters are often used interchangeably to explain the same growth phenomenon. The sensitivity and specificity of each anthropometric indicator are a function of the cutoff value selected and dictate which infants will be classified as having FTT and which infants will be classified as healthy. Depending on the sensitivity and specificity of the indicator, some infants with FTT will be classified as healthy, and some healthy infants will be classified as having FTT. A clear rationale for the selection of an anthropometric indicator and a cutoff value for defining FTT are important for increasing the generalizability of research findings and thereby expanding the current knowledge base related to FTT.