I have almost finished writing this book and have gone over many drafts. I am happy to still have a sense of wonder when reading the chapters. The wonder comes because networks of thousands of interacting components are generally incomprehensible. There is no a priori reason that immensely complex biological systems would be understandable. But despite the fact that biological networks evolved to function and not to be comprehensible, simplifying principles can be found that make biological design understandable to us.