Proportionality is the branch of knowledge that analyses the relationship between different parts of the body or between the body and some reference to determine their relative size. There are two main types of proportionality: body indices and comparison with a reference model. Within the body indices, we find: (a) relative measurements or proportions, such as the relative sitting height, relative arm span, relative upper limb length, relative lower limb length, and waist-to-height index; (b) indices such as the intermembral index, brachial index, acromial-iliac index, crural index, thoracic index, waist-hip index and fat distribution index; (c) ponderal indices or robustness indices, highlighting the Quetelet index or body mass index, Bouchard index, and ponderal index. This chapter explains what each one consists of, the formula to calculate them, and how to interpret their results. Next, the comparison with the reference model, also known as Phantom model, is presented. This model was created to compare an individual’s anthropometric variables with those of a reference population. It specifies how to calculate the Phantom model value step by step, as well as how to interpret it.