The technique of allometry investigates the effects of size on such variables as food intake, energy requirements, growth rates and age at first reproduction. This book brings together much of what is known about the consequences of size and provides a new and mathematically rigorous framework within which many quantitative predictions are made and tested using published and unpublished data. Explanations are proposed for many previously unexplained phenomena such as why in some species females are thousands of times heavier than males, whereas in no species are males more than about eight times heavier than females. The models presented afford a synthesis of the effects of size and open up pathways for further theoretical investigation and experimental testing. Care has been taken to give verbal presentations of all the mathematical conclusions to ensure that the text is widely intelligible.