  This paper investigates the role of size in biological organisms. More
specifically, how the energy demand, expressed by the metabolic rate, changes
according to the mass of an organism. Empirical evidence suggests a power-law
relation between mass and metabolic rate, namely allometric law. For vascular
organisms, the exponent $\beta$ of this power-law is smaller than one, which
implies scaling economy; that is, the greater the organism is, the lesser
energy per cell it demands. However, the numerical value of this exponent is a
theme of an extensive debate and a central issue in comparative physiology. It
is presented in this work some empirical data and a detailed discussion about
the most successful theories to explain these issues. A historical perspective
is also shown, beginning with the first empirical insights in the sec. 19 about
scaling properties in biology, passing through the two more important theories
that explain the scaling properties quantitatively. Firstly, the Rubner model,
that consider organism surface area and heat dissipation to derive $\beta =
2/3$. Secondly, the West-Brown-Enquist theory, that explains such scaling
properties as a consequence of the hierarchical and fractal nutrient
distribution network, deriving $\beta = 3/4$.
