Plants have evolved sensory mechanisms to detect pathogen attack and trigger signalling pathways that induce rapid defence responses. These mechanisms include not only direct detection of pathogen-derived elicitors (e.g. pathogen-associated molecular patterns (PAMPs) and avirulence factors or effectors) but also indirect sensing of pathogens' impact on the host plant. Among the first plant barriers to pathogen ingress are the cell wall and the cuticle. For those pathogens that penetrate the plant cell wall to gain access to water and nutrients of the plant protoplast, small wounds at penetration sites are created by enzymatic or physical disruption of the plant cell wall. Thus, cell wall integrity sensing is one mechanism by which plants may detect pathogen attack. Some plant cell wall fragments, notably oligogalacturonic acids, elicit similar defence responses in plants as the non-specific PAMP elicitors (e.g. production of reactive oxygen species, elevated expression of defence-associated genes), suggesting that PAMP signalling may provide a good model for studying cell wall integrity sensing in plants. However, much remains to be discovered about this sensing mechanism.