The gut microbiome plays a fundamental role in regulating pig health and growth. Understanding the functions performed by the microbiome is vital when considering it as a target to improve pig health and growth, a pursuit driven by the increasing regulation of traditional means of disease control and growth promotion. This chapter explores the structure, diversity and functions of the pig gut microbiome, focusing on the role of the resident bacterial communities. It examines their relationships, interactions, and contributions to the host, ranging from the production of antimicrobial substances and prevention of pathogen colonisation to improvement of nutrient digestibility and the production of volatile fatty acids (VFAs) and vitamins. The chapter also reviews bacterial communication and the antibiotic resistome of the pig gut, outlining how they may be targeted/manipulated to reduce antibiotic resistance and promote improved gut health.