The following PhD thesis examines the crucial role of Wastewater Treatment Plants (WWTPs) and grasslands in the spread and maintenance of antibiotic-resistant bacteria (ARB) and antibiotic resistance genes (ARGs) via a One Health context. Through an in-depth investigation of two urban WWTPs in Ireland, the research sheds light on the prevalence and mechanisms of antibiotic resistance among faecal coliforms. Results revealed that more than 90% of isolated faecal coliforms exhibited resistance to common antibiotics like amoxicillin and ampicillin, with a significant proportion being multidrug-resistant. Notably, the study discovered a substantial presence of extended-spectrum β-lactamase (ESBL) producing isolates, indicating a high potential for horizontal gene transfer, which could contribute to the spread of antibiotic resistance. Complementing this research, the study provides a novel exploration of the global scale of antimicrobial resistance (AMR) plasmids. These plasmids isolated from chicken caecum, containing combinations of known AMR genes, metal resistance genes, virulence factors, and replicon types, were detected across 63 countries in diverse environments and species, including humans, animals, and the environment. This compelling evidence underscores the urgent need for expanded global surveillance and sequencing of AMR plasmids to better understand and combat the spread of antibiotic resistance. Furthermore, the research delves into the often-overlooked role of grasslands within the environmental-animal nexus of AMR. Contrary to common focus areas such as soil or vegetables, our data suggests grasslands hold a more diverse and abundant reservoir of ARGs and mobile genetic elements (MGEs). These findings have critical implications for human and animal health, particularly in the context of grazing food animals. Lastly, through the characterisation of 173 plasmids isolated from the two Irish WWTPs, the research unveils a globally interconnected evolutionary history and an extensive prevalence of multidrug-resistant (MDR) plasmids. The results further confirm the significant role WWTPs play in the distribution and persistence of AMR, highlighting the importance of these environments in the development of strategic AMR control and the necessity for a comprehensive, globally-linked surveillance system.