Microalgae are microscopic algae, typically found in freshwater and marine systems. Microalgae, capable of performing photosynthesis, are important for life on earth; they produce approximately half of the atmospheric oxygen and use simultaneously the greenhouse gas carbon dioxide to grow photoautotrophically. The biodiversity of microalgae is enormous and they represent an almost untapped resource. In this book, the authors present current research in the study of microalgae, including microalgal biotechnological applications in nutrition, health and the environment; using microalgae biomass for biodiesel and biofuel production and microalgae for aquaculture.