Our planet faces many challenges. In 2013, an international partnership of more than 140 scientific societies, research institutes, and organizations focused its attention on these challenges. This project was called Mathematics of Planet Earth and featured English- and French-language blogs, accessible to nonmathematicians, as part of its outreach activities. This book is based on more than 100 of the 270 English-language blog posts and focuses on four major themes: A Planet to Discover; A Planet Supporting Life; A Planet Organized by Humans; and A Planet at Risk.