This book is about the importance of animal size. Although it is informative and tells a great deal about what we know today, it is not encyclopedic. It should therefore be easy to read and use.

When we try to find the rules that govern animal function, we tend to think in terms of chemistry. We think of water, salts, proteins, enzymes, oxygen, energy, and so on – a whole world of chemistry. We should not forget that physical laws are equally important; they determine rates of diffusion and heat transfer, transfer of force and momentum, the strength of structures, the dynamics of locomotion, and so on. Physical laws provide possibilities and opportunities, yet they impose constraints and set limits to what is physically possible. It is our purpose to understand these rules because of their profound implications when we deal with organisms of widely different size and scale.

The book requires a minimum of basic numerical skills. It turns out that body size relationships are best expressed with logarithms and powers. These are simple enough to require familiarity with no more than a few easy algebraic operations. The entire book has been made as simple as possible, and most arguments are intuitively understandable. Although I use equations, which are useful for calculations, the conclusions are presented verbally, so that a minimum of effort is required to read the book from cover to cover.