Microbes live in diverse, challenging and competitive environments. To survive and propagate microbes must be able to sense and respond to environmental fluctuations, such as changes in pH, nutrient status or temperature. As such, bacteria have a number of signal transduction mechanisms at their disposal that allow them to detect a range of different stimuli, integrate different signals and react to them appropriately. The work presented in this thesis aimed to understand more about the signalling cascades that the Gram positive soil-dwelling bacterium <em>Bacillus subtilis</em> uses to mediate its transition from a motile lifestyle that requires rotating helical flagella, to a sessile lifestyle called the biofilm, where cells adhere to a surface and are encased in a self-produced extracellular polymeric matrix. Bacterial tyrosine phosphorylation is required for <em>B. subtilis</em> biofilm formation and has been suggested to also play a role in regulating the putative motility protein, YvyG. This led to the hypothesis that tyrosine phosphorylation might play a role in both motility and biofilm formation. The first part of this thesis investigates this hypothesis and successfully ascribes a function to YvyG as an orthologue of a flagellar type 3 secretion system chaperone that is essential for flagellar assembly. Crucially this work provides further evidence that the <em>B. subtilis</em> flagellum is regulated by both conserved and species-specific means. These experiments led to YvyG being re-named as FlgN. Despite previous work suggesting that phosphorylation of YvyG was important for protein function and localisation, the data presented here found no evidence of this, and therefore indicate that the impact of bacterial tyrosine phosphorylation must be assessed in vivo before any significance can be drawn from the identification of such modifications by in vitro approaches. The second part of this study examines the role of the DegS-DegU two component signal transduction system in mediating the transition from motility to biofilm formation. DegS-DegU is required for both motility and biofilm formation, and previous work indicated that DegS-DegU may sense flagellar assembly. The data presented show that upon an inhibition of flagellar rotation DegU~P levels are increased, as inferred from accepted proxies. This could conceivably be the first step in biofilm formation to allow cells to sense and respond to a surface and change their gene expression profile. The <em>B. subtilis </em>flagellum therefore acts as a mechanosensor to control the DegS-DegU two component system. Collectively, the work presented here contributes to our understanding of how <em>B. subtilis</em> regulates flagellar assembly, and further enhances our knowledge of how bacteria are able to use their flagella not only as devices for propulsion, but also to sample changes in the extracellular environment.