The intestinal microbiota and its functions are regarded as critical for host health and disease. Probiotics can influence the gut microbiome and its interactions with the host, and are currently defined as “live microorganisms that, when administered in adequate amounts, confer a health benefit on the host”. Probiotics have become common components of strategies to promote livestock health, welfare and productivity, not least due to restrictions on the use of antimicrobial drugs. Common probiotic organisms are considered commensals and are ‘generally recognized as safe’ (GRAS) via oral administration. This review outlines potential probiotic mechanisms, including recent findings. These mechanisms include those interactions primarily occurring between the supplemented probiotic microorganisms and the indigenous intestinal microbiota, perhaps within the gut lumen, as well as more direct interactions with the host via mucosal receptors or more distally following absorption of microbial components. There is good evidence that the gut microbiome is relatively stable in ‘healthy’ individuals and resistant to ‘colonisation’ by exogenous microbes, which helps exclude pathogens, but has implications for the establishment of probiotics, and could increase the importance of microbe-microbe interactions. However, such microbiomes may be receptive to complementary microbes or functions, while supplemented probiotics may dominate luminal populations, particularly in less populated regions of the intestine. Moreover, host-adapted microbes or microbiomes may elicit different host responses and/or be more effective. Some considerations for the interpretation of study results, including extrapolation from different models or microbial strains, are also included. In addition, notable mechanistic and/or pathogen challenge studies from pigs and poultry are highlighted to underline the recognised potential of probiotics in these species, particularly as the appropriate selection of microorganisms and their application continues to be better understood and improve.