Plant innate immunity is a potential surveillance system of plants and is the first line of defense against invading pathogens. The immune system is a sleeping system in unstressed healthy plants and is activated on perception of the pathogen-associated molecular patterns (PAMP; the pathogen’s signature) of invading pathogens. The PAMP alarm/danger signals are perceived by plant pattern-recognition receptors (PRRs). The plant immune system uses several second messengers to encode information generated by the PAMPs and deliver the information downstream of PRRs to proteins which decode/interpret signals and initiate defense gene expression. This book describes the most fascinating PAMP-PRR signaling complex and signal transduction systems. It also discusses the highly complex networks of signaling pathways involved in transmission of the signals to induce distinctly different defense-related genes to mount offence against pathogens.