Though they appear at the opposite end of the spectrum, being overweight and being malnourished (and thus underweight) share characteristics in common. Primary among these shared characteristics is that both conditions increase children's risk of developing serious diseases. Both conditions can also negatively affect children's physical, emotional, and social development.

Whether a child is overweight, underweight, or of normal weight is always something that needs to be judged in context of that child's height, age and sex. In other words, older, taller children are supposed to weigh more than younger, shorter children. It does not make sense to consider weight in isolation of these other factors.

Body Mass Index (BMI) is a statistic (a calculated measurement) that summarizes children's height and weight in the context of their age and sex. Doctor can measure children's BMI in the office. Alternatively, online children's BMI calculators are available that can compute BMI based on children's height, weight, age and sex. The BMI number can be used to classify children into categories including 'underweight', 'healthy weight', 'at risk for being overweight', and 'overweight'.