The innate immune system comprises the cells and mechanisms that defend the host from infection by other organisms, in a non-specific manner. This means that the cells of the innate system recognise, and respond to, pathogens in a generic way, but unlike the adaptive immune system, it does not confer long-lasting or protective immunity to the host. Innate immune systems provide immediate defence against infection, and are found in all classes of plant and animal life. The innate system is thought to constitute an evolutionarily older defence strategy, and is the dominant immune system found in plants, fungi, insects, and in primitive multi-cellular organisms. This book gathers the latest research in the field.