Brassica vegetables, such as broccoli, are characterised by the presence of sulphur-containing compounds, termed glucosinolates, which are associated with potential health benefits for humans. Glucosinolates are metabolised in the gut by members of the gut microbiota, producing biologically active breakdown products, such as isothiocyanates. The effects of consuming Brassica on the composition of the gut microbiota, and the bacterial mechanisms employed for glucosinolate metabolism, are unclear, and forms the basis of the research presented in this thesis. Culturing human faecal microbiotas in an in vitro batch fermentation model identified the bacterial-mediated reduction of glucoraphanin and glucoiberin to glucoerucin and glucoiberverin, respectively. An Escherichia coli strain was found to exhibit reductase activity on glucoraphanin and the broccoli-derived compound S-methylcysteine sulphoxide, through the reduction of the sulphoxide moiety. Within this fermentation model, the relative proportions of members of the genus Lactobacillus were found to significantly increase when the microbiota was repeatedly exposed to a broccoli leachate, and 16S rDNA sequencing identified these as L. fermentum. Metabolite analysis detected relatively high concentrations of lactate and short-chain fatty acids when faecal microbiotas were cultured in the presence of broccoli leachates, compared to a glucose control media. A human dietary study investigating the effects of Brassica on the microbiota composition revealed a significant association between dietary Brassica and changes to the relative proportions of a number of bacteria, many of which belong to the Clostridiales. Further studies are required to reveal the nature of this association, and whether the presence of glucosinolates may have been a factor. The work presented in this thesis highlights the strong connection between diet, the gut microbiota, and the potential health benefits to the host that may be derived from the bacterial metabolism of dietary compounds.