All animals, including humans, derive from a single cell, which possesses all the genetic instructions needed to define how the animal will look like. However, during development, the millions of cells that derive from the zygote will only select part of this genetic information to give rise to the various organs of the body. The coordination of different cell behaviours during development results in the formation of specialized tissues and organs giving rise to highly adapted animals. This book provides an overview of how this diversification is achieved during organ formation and how it may have evolved. Conserved cellular processes are presented using examples from selected vertebrate and invertebrate species that illustrate how developmental biologists are solving the complex puzzle of organ formation. This volume is aimed to students, researchers and medical doctors alike who want to find a simple but rigorous introduction on how gene networks control organ formation.