© Springer Science+Business Media New York 2016. It has been well known that oligosaccharides generated from fungal, bacterial, or plant cell walls can induce various defense responses in plant cells. Traditionally they have been called as “elicitors” or “general elicitors” that induce defense responses in a wide range of plant species. These molecules are now classifi ed as representative MAMPs/PAMPs (microbe/pathogen-associated molecular patterns) or DAMPs (damage/danger-associated molecular patterns) and believed to play important roles in plant immunity, as a trigger of the so-called pattern- triggered immunity (PTI). Biological activities of these oligosaccharide elicitors, including structure/function relationships, corresponding receptors, and downstream signaling, have been the subject of intense studies, by which these oligosaccharides became most well-characterized oligosaccharins. In this chapter, present knowledge on the plant receptors of carbohydrate elicitors generated from the cell walls of pathogenic microbes and also host plants is summarized. Mechanism of the activation of downstream signaling is discussed for chitin receptor, as this system has become the most well-characterized oligosaccharin receptor in plant immunity. It is also discussed how pathogenic microbes try to escape from the MAMP- mediated detection of their invasion by host plants.