Plants possess a sophisticated immune system that allows them to detect and respond to potential pathogens present in their environment. Plants sense pathogens through microbe-associated molecular patterns (MAMPs). MAMPs are conserved structures that are vital for the pathogen but absent in the host plant. When a MAMP binds to its cognate receptor, a signalling cascade is initiated inside the cell, which leads to the activation of a number of defence responses that enables the plant to fight the invading pathogen. Thus, pattern-triggered immunity (PTI) is established. Lipopolysaccharide (LPS) is an amphiphilic molecule present in the outer membrane of Gram-negative bacteria. It functions as a protective barrier against harsh environments and antimicrobial compounds. LPS has long been known to act as a MAMP in plants, and it induces a range of defence responses. This study explores various aspects of the role of LPS as a MAMP in plant-microbe interactions. The plant pathogenic bacterium Xanthomonas campestris pv. campestris (Xcc) was shown to change the structure of its LPS when growing inside Arabidopsis thaliana. This might be a mechanism employed by Xcc to avoid immune recognition, as has been shown in mammalian pathogens. Furthermore, the vesicle trafficking protein SYP121 was found to be involved in the immune response induced by the two structurally very different MAMPs Xcc LPS and flg22. Localization of Xcc LPS in planta was examined as well. The structure and function of LPS from two strains of Xanthomonas citri pv. citri (Xac) were investigated in order to elucidate the mechanisms behind differing host ranges. LPS from both Xac strains was shown to be active MAMPs in Arabidopsis thaliana. However, more studies are needed to clarify why Xac strains have different host ranges. LPS of the symbiotic bacterium Bradyrhizobium sp. BTAi1 possesses the unique bicyclic monosaccharide bradyrhizose and is not recognized as a MAMP by plants. Synthetic bradyrhizose oligomers and enantiomers were prepared and tested in planta, and mostly found to be immunologically silent in Arabidopsis thaliana, indicating that Bradyrhizobium sp. BTAi1 has developed the unusual LPS structure to facilitate symbiosis. However, some of the unnatural enantiomers did induce a defence response, underlining the importance of LPS structure in recognition.