In plant innate immunity, the first line of microbial recognition leading to active defence responses relies on the perception of pathogen-associated molecular patterns (PAMPs) by pattern-recognition receptors (PRRs). This recognition leads to PAMP-triggered immunity (PTI). Despite the numerous PAMPs recognised by plants, only a handful of PRRs are characterised. For most, they correspond to transmembrane proteins with a ligand-binding ectodomain. PRRs interact with additional transmembrane proteins that act as signalling adapters or amplifiers to achieve full functionality. The crucial role of PRRs in anti-microbial immunity is demonstrated by the direct targeting of PRRs and their associated proteins by pathogenic virulence effectors.