A forest is for many people something obvious: the domain of trees, as opposed to grass landscapes such as lawns, meadows, savannahs, and steppes, identified on every continent. A tree is undoubtedly the backbone of a forest. The notion of trees is problematic as to size, age, shape, and biology. The number of plant species present on the surface of the planet is still questionable and a fortiori that of trees. The total number of trees on the planet is another subject of assessments resulting in very different but impressive numbers. Numbering trees is, however, misleading because it depends on the parameters and categories of size, height, or diameter used. There are forests all around the globe, from the northern polar circle to the equator. Various forest types develop in the subtropical crown, and finally, forests are also diffusely present in drylands.