Campylobacter jejune is the leading cause of gastroenteric bacterial infectious disease in the EU with the majority of human cases being identified as from a poultry source. If the number of cases is to be reduced, more needs to be understood about the behaviour of C. jejuni in the poultry host and its wider epidemiology. Multiple genotypes can be found within anyone C. jejuni population, potentially allowing for quick genotypic and therefore phenotypic adaptation to changes in environment. It was hypothesised that the relatively high level of genetic instability observed in C. jejuni may allow it to change its surface epitopes over the course of colonisation. A five week chicken colonisation study observed changes inflaA that conesponded to decreased reactivity with chicken antibodies at five weeks compared to two weeks. A Multi-Locus Sequence Typing system developed for the characterisation of C. jejuni strains groups sequence types into clonal complexes (CCs). It has become apparent that whilst some CCs contain strains isolated from a range of sources (host genera lists), others contain a majority of strains isolated from only one host group (host specialists). The link between these epidemiological observations and in vivo phenotypes is unknown. The results of bird colonisation work suggest it is not as simple as chicken specialists being better adapted to colonising chickens than generalists or other host specialists. It is not known whether this is because the apparent host association reflects pressures other than colonisation ability in the host, or because the multiple genotypes that are present in any C. jejuni population mean there are still variants within a non-chicken specialist population that are fit for colonising chickens to high levels. In conclusion, these results show that genetic (and therefore antigenic) instability in C. jejuni contributes to its ability to colonise poultry.