Plants are called upon to defend themselves against a variety of organisms (bacteria, protists, fungi, insects and vertebrates) that use them as a source of nutrients. Plant immunity is the innate or induced ability of plants to detect and counteract invasive species before they can inflict serious harm. Molecules emitted by pathogens are perceived by receptors at the surface of or inside plant cells. This perception induces complex signaling cascades leading to a defensive response to pathogen attack. This chapter gives an overview of the most important defense strategies in higher plants.