The Gulf of Guinea oceanic islands (Príncipe, São Tomé, and Annobón) are among the most endemic-rich regions of the planet. Historical scientific studies of the islands’ unique biodiversity are scattered in a variety of publications, many of which are difficult to access. More recently, there has been a growing interest in the islands, which is reflected in a burst of new studies, reports, and ongoing projects. Here we aim to provide an updated and comprehensive synthesis, covering all the key information and references on the biodiversity of these islands. The goal of the book is to be a comprehensive reference for students, researchers, and conservationists dedicated to the study and preservation of this unique biodiversity. It also intends to serve as a basis for local stakeholders to make informed decisions, namely regarding conservation actions. The book is divided into three main sections: (1) a general overview of the islands and their biodiversity, including aspects of natural and human history (six chapters); (2) detailed accounts on different taxonomic groups (16 chapters); and (3) the conservation, environmental education, and research challenges that lie ahead (three chapters).