Microalgae, a fascinating world of microscopic and photosynthetic beings. Being an integral part of aquatic ecosystems, they have an important role in the feeding of aquatic species, in oxygenation by their photosynthesis and in the balance of ecosystems. With the participation of colleagues, this book is a collection of the main species of microalgae of Lake Sidi Mohamed Benali, Wilaya Sidi Bel Abbes. It offers to the passionate readers, teachers and students, a precise documentation of the microscopic world of the brackish waters of this lake.