If trees are our partners in the hasardous entreprise that is life on Earth, is it not because we share common interests in light and water, soil fertility, space and warmth with trees ? What are trees ? How can we define this extremely ancient life form that has such a singular architecture and capabilities for great prowess ? The author is a botanist and biologist, specialized in the architecture of trees and of the ecology of tropical rain forests. Keeping good distance from anthropomorphism, the present contribution attempts to lay the foundations of a structural and functional analysis of trees. The text is extracted from his eponymous 2006 book published under the French version of this title by Actes Sud publishers.