Plants detect microbial threats through pattern-recognition receptors (PRRs) that recognize pathogen-associated molecular patterns (PAMPs). In Arabidopsis thaliana, FLS2 and EFR are PRRs that recognize flagellin and elongation factor Tu (EF-Tu), respectively, and contribute to resistance against bacterial pathogens. BAK1, a leucine-rich repeat receptor-like kinase, is involved in signaling by FLS2 and EFR. bak1 mutants show reduced sensitivity to flagellin and EF-Tu, but not to brassinosteroids. FLS2 and BAK1 form a complex in vivo in a ligand-dependent manner, indicating that BAK1 plays a role in PRR-dependent signaling, which initiates innate immunity. BAK1 is also involved in BRI1 signaling, a receptor for brassinosteroids. The study shows that BAK1 is a positive regulator of PAMP signaling in Arabidopsis, and that it interacts with FLS2 in a ligand-dependent manner. The complex formation between FLS2 and BAK1 is specific for flagellin and does not occur after stimulation with elf26 or brassinolide. BAK1 is also involved in the endocytosis of FLS2. The study highlights the role of BAK1 in plant defense and signaling, similar to the TOLL receptor in Drosophila, which controls both development and immunity. BAK1 has at least two roles in plants: as a positive regulator of PAMP receptors and as a regulator of the plant hormone receptor BRI1. The study provides evidence that BAK1 is involved in the signaling pathway that initiates innate immunity in plants.Plants detect microbial threats through pattern-recognition receptors (PRRs) that recognize pathogen-associated molecular patterns (PAMPs). In Arabidopsis thaliana, FLS2 and EFR are PRRs that recognize flagellin and elongation factor Tu (EF-Tu), respectively, and contribute to resistance against bacterial pathogens. BAK1, a leucine-rich repeat receptor-like kinase, is involved in signaling by FLS2 and EFR. bak1 mutants show reduced sensitivity to flagellin and EF-Tu, but not to brassinosteroids. FLS2 and BAK1 form a complex in vivo in a ligand-dependent manner, indicating that BAK1 plays a role in PRR-dependent signaling, which initiates innate immunity. BAK1 is also involved in BRI1 signaling, a receptor for brassinosteroids. The study shows that BAK1 is a positive regulator of PAMP signaling in Arabidopsis, and that it interacts with FLS2 in a ligand-dependent manner. The complex formation between FLS2 and BAK1 is specific for flagellin and does not occur after stimulation with elf26 or brassinolide. BAK1 is also involved in the endocytosis of FLS2. The study highlights the role of BAK1 in plant defense and signaling, similar to the TOLL receptor in Drosophila, which controls both development and immunity. BAK1 has at least two roles in plants: as a positive regulator of PAMP receptors and as a regulator of the plant hormone receptor BRI1. The study provides evidence that BAK1 is involved in the signaling pathway that initiates innate immunity in plants.