Failure to thrive (FTT), also known as weight faltering or faltering growth, indicates insufficient weight gain or absence of appropriate physical growth in children. FTT is usually defined in terms of weight, and can be evaluated either by a low weight for the child's age, or by a low rate of increase in the weight.

The term "failure to thrive" has been used in different ways, as there is no objective standard or universally accepted definition for when to diagnose FTT. One definition describes FTT as a fall in one or more weight centile spaces on a World Health Organization (WHO) growth chart depending on birth weight or when weight is below the 2nd percentile of weight for age irrespective of birth weight. Another definition of FTT is a weight for age that is consistently below the 5th percentile or weight for age that falls by at least two major percentile lines on a growth chart. While weight loss after birth is normal and most babies return to their birth weight by three weeks of age, clinical assessment for FTT is recommended for babies who lose more than 10% of their birth weight or do not return to their birth weight after three weeks. Failure to thrive is not a specific disease, but a sign of inadequate weight gain.

In veterinary medicine, FTT is also referred to as ill-thrift.