Within the pig population of Northeast Scotland resistance to tetracycline and ampicillin was found to be widespread among commensal Escherichia coli, as also noted among anaerobic commensals of the genera Bacteroides and Lactobacillus. E. coli resistant to apramycin and nalidixic acid and enterococci resistant to vancomycin were less frequently isolated but remained common. Production system, piglet age and recent transportation were found to influence the antibiotic resistant proportion of the faecal E. coli population whilst dietary copper inclusion did not. Within the intestinal tract higher proportions of resistance to ampicillin were noted among the E. coli of the ileum and caecum than more distal sections and to apramycin and nalidixic acid among the ileal mucosal associated E. coli than those of the ileal lumen. Novel techniques for characterising and modelling antibiotic resistant bacterial populations were suggested and investigated. A spiral-plater system was used to investigate the phenotypic expression of antibiotic resistance among faecal E. coli, complemented by the development of a PCR technique to identify the genetic basis of tetracycline resistance. The effects of antimicrobial supplementation on the indigenous enteric flora were monitored through the validation and use of an in vitro porcine ileal simulation. Within the same model transmission of genetic material encoding antibiotic resistance was demonstrated between commensal and pathogenic representatives of the Enterobacteriaceae under ileal conditions. Variation in the mucosal adherence of E. coli expressing different antibiotic resistant attributes was investigated in vitro following the implementation of a modified cell culture technique.