The term innate immunity has been described as '. . . the surveillance system that detects the presence and nature of the infection and provides the first line of host defense . . .' (Medzhitov, 2001; Nat Rev Immunol 1: 135-145). The strategy of innate immunity is based on the recognition of constitutive and conserved molecules from pathogens by specific receptors, triggering defence responses (Medzhitov and Janeway, 2002; Science 296: 298-300). It has been only within the past few years that studies of plant innate immunity, especially in Arabidopsis, have provided important insights into molecular details that define innate immunity in plants. Here we review the innate immune response in Arabidopsis, where leucine-rich repeat (LRR) cell surface receptors play central roles in monitoring the presence of pathogen (microbe) associated molecules to initiate the rapid expression of defence genes. The PAMPS also activate the expression of genes encoding a family of endogenous peptides (AtPep1 paralogues) and their receptor (PEPR1) that amplify defence signalling through a feedback loop initiated by PAMPS. The concept of innate immunity has provided a valuable framework for researchers to re-evaluate the roles of exogenous and endogenous signals that regulate the expression of plant defensive genes.