Plants are able to detect microbes by pattern recognition receptors in the host cells that, upon recognition of the enemy, activate effective immune responses in the invaded tissue. Recognition of microbes occurs by common conserved structures called microbe-associated molecular patterns (MAMPs). Plant pathogens and beneficial soil-borne microbes live in close contact with their host. Hence, prevention of the host's defence programme is essential for their survival. Active suppression of host defences by microbial effector proteins is a well-known strategy employed by many successful plant-associated microbes. Evasion of host immune recognition is less well studied but is emerging as another important strategy. Escape from recognition by the host's immune system can be caused by alterations in the structure of the recognized MAMPs, or by active intervention of ligand-receptor recognition. This paper reviews the structure and recognition of common MAMPs and the ways that plant-associated microbes have evolved to prevent detection by their host.