Rising global concern about antimicrobial resistance (AMR) has drawn attention to the use of antibiotics in livestock. Of particular concern is the excessive, sub-therapeutic use of antibiotics for disease prevention, in particular the Critically Important Antimicrobials (CIA), which are reserved as a last resort for the most severe and resistant human infections. Understanding current usage of antibiotics in these animals is essential to design and implement effective interventions that will reduce unnecessary antibiotic use. However, to date few studies have assessed the use of antibiotics for pig production in low- and middle-income countries. The aim of the thesis is to investigate patterns of antibiotic use and determinants influencing antibiotic use in pig production in Thailand in order to contribute to the development of policies to optimise the use of antibiotics in pig production and control AMR. Two systematic reviews on antibiotic use in pig production and its associations; and methods and measurements for quantification of the use of antibiotics in pig production were conducted to explore the antibiotic use in pig production globally. Using mixed methods for the empirical research, a survey of pig farmers (n=84), a survey of feed mills (n=31), and interviews with veterinarians (n=5) were undertaken to understand the patterns of antibiotic use. A total of 31 in-depth interviews were conducted with different categories of actors: pig farmers (n=13), drug retailers (n=5), veterinarians (n=7), government officers (n=3) and representatives of animal and human health associations (n=2) to explore determinants influencing antibiotic use in pig production in Thailand. Evidence revealed several practices associated with antibiotic use in pig production in Thailand, which may contribute to the emergence and threat of AMR to people including a high proportion of pig farmers using antibiotics for disease prevention and using antibiotics in the CIA category; and a large volume of antibiotics being administered in the form of medicated feed. The multi-faceted nature of the views and practices may contribute to misuse or overuse of antibiotics in the study locations, including misconceptions about the nature of antibiotics and AMR (particularly among smallholders), lack of facilities and financial means to establish an antibiotic-free farm, lack of sufficient training on AMR and antibiotic prescribing for veterinarians, the profit motive of pharmaceutical companies and their ties to farm consultants, and lack of sufficient regulatory oversight. Given the concern on the loss of antibiotic effectiveness through the development of AMR, collective action is required to improve the practices of all actors towards the optimisation of antibiotic use in pig production. For example, farmers need better access to veterinary services and reliable information about animal health needs and antibiotics. Further development of professional training and clinical guidelines, and the establishment of a code of conduct, would help improve antibiotic dispensing practices. In addition, a combination of market access rules by the private sector and control through regulations such as establishing veterinary antibiotic prescriptions monitoring systems and limiting the pharmaceutical industry’s influence on the farmers’ and veterinarians’ decision on the use of antibiotics, could be an effective instrument to govern the use of antibiotic in livestock production.