In the last decades, knowledge about the biology of the Amazon river dolphin Inia geoffrensis has increased considerably. Accordingly, we present a review of the published literature on this river dolphin, with a focus on topics with the most advancements and those that are important to the species conservation. We also present new information, including emerging knowledge on the other species of Inia. We summarize the current knowledge of the biology of Inia, covering aspects of taxonomy, distribution, life history and reproduction, genetics, anatomy and physiology, health, behavior, ecology, demographic parameters, and population trends. We also highlight the main anthropogenic threats affecting the species and the resulting conservation efforts.