This paper aims to guide readers embarking on the complex literature in respect of childhood and adolescent obesity. It opens with a discussion of definitions of 'obesity' based on overall fat levels and the significance of fat distribution. This is followed by simple descriptions of the various techniques used to measure fat, including density-based, scanning, bioelectrical impedance and anthropometric methods. The paper then turns to 'overweight' and the measurement of weight in relation to height, particularly via body mass index (BMI). While it is a relatively simple measure and a valuable tool, BMI has several disadvantages, which are described. These include a lack of consensus on which values should be used to define 'overweight' or 'obese', with the result that the literature contains a confusing multiplicity of child and adolescent obesity rates.