This thesis is an investigation of drug use practices of Ontario sheep producers, which was not previously researched. Public health concerns surround drug use in food-producing animals, including drug residues and antimicrobial resistance. A study was initiated which prospectively collected drug use data over 12 months from 49 Ontario sheep farms. Veterinary drug use with focus on antimicrobial use (AMU), extra-label drug use (ELDU) and their determinants is described. Fifteen drug categories were recorded, representing 2,715 treatment events. Antimicrobials, vitamin and mineral supplements, biologicals and endectocides were used most frequently. Antimicrobials with high mean exposure rates included penicillins and oxytetracycline. Rates of using unlicensed antimicrobials was high, as was ELDU of licensed antimicrobials. However, diseases treated most often were not associated with higher rates of AMU or ELDU. Results are useful in developing drug use and licensure strategies for the Canadian sheep industry.