Since the start of the previous century, antibiotics have been among our most important medicines, and a cornerstone in both human and veterinary medicine. However, the last decades, resistance against antibiotics have risen to become a clinical problem, and has been emphasized by health organizations as one of the leading medical challenges in the 21st century. It is therefore important to explore factors that contribute to the development of resistance. Overuse and misuse of antibiotics can promote development of resistance. There has been much focus on use of antibiotics in human medicine. Meanwhile, millions of tons of antibiotics are used in agriculture world-wide. Some of this amount is used for healthy animals, such as use of antibiotics as growth promoters. The aims of this study have been to review how the use of antibiotics in agriculture affects resistance in production animals and farm environments, and if this use has implications for antibiotic resistance in bacteria that affect humans.