Bacteriocins are potent antimicrobial peptides produced by bacteria, with diverse structures and functions. They have been studied for nearly a century and are used as food preservatives. Many bacteriocins have complex post-translationally modified structures, while others are simple linear peptides. These peptides are amenable to bioengineering and heterologous expression, enabling the discovery and modification of novel antimicrobials. The global antimicrobial resistance crisis demands new therapeutics to combat infectious pathogens. Bacteriocins, which can be broad or narrow spectrum, are promising tools for this purpose. However, few have progressed beyond preclinical trials. This review explores the diversity of bacteriocin structure and function, methods for identification and optimization, and the reasons for limited translation. Bacteriocins offer pharmacological advantages such as activity against pathogens in the nanomolar and picomolar ranges, narrow spectra, general non-toxicity, and distinct mechanisms of action. The diversity in bacteriocin structures and mechanisms also represents a range of ribosomally produced antimicrobials that are amenable to genetic engineering strategies enabling the production of bioengineered variants with superior efficacy against target microorganisms. Bacteriocins are natural agents of microbial competition, offering an advantage to bacteria over sensitive neighboring species, enabling invasion, colonization, and protection of a niche in complex environments such as the gut microbiome. By exploiting these natural mechanisms, they can be used as a tool for humans to deliver antimicrobial activity as pure peptides or composite nanoparticles or can be codified within live biotherapeutics for in situ expression in microbiomes. Advances in microbiome metagenomics and other 'omics' technologies have revealed catalogues of genome-mined and metabolome-mined bacteriocins, with new families continually being discovered. However, discovery is outpacing characterization and application. Narrow-target spectrum bacteriocins are increasingly desirable as we advance our understanding of host-microbiota interactions. Bacteriocins have been found to have a diverse set of other biological functions, including antiviral, cytotoxic, and immune-modulating activities. They can also modulate immune systems, promote plant growth, and support gene acquisition. Bacteriocins are typically cationic hydrophobic peptides, many of which are active at negatively charged bacterial cell surfaces. Their activities once they reach the cell surface are diverse, including specific interactions with cell-wall or cell-membrane components, general membrane interaction, pore formation, or receptor-mediated cell penetration and interference with intracellular processes. Nisin is well established to have a dual mechanism of action, binding the cell-wall precursor molecule lipid II, thereby inhibiting peptidoglycan biosynthesis and subsequent pore formation leading to cell death. Numerous post-translationally modified bacteriocins share a conserved nisin-like N-terminal lipid II binding domain but do not possess the same C-terminal domain and do not form pores. Many bacteriocins do not targetBacteriocins are potent antimicrobial peptides produced by bacteria, with diverse structures and functions. They have been studied for nearly a century and are used as food preservatives. Many bacteriocins have complex post-translationally modified structures, while others are simple linear peptides. These peptides are amenable to bioengineering and heterologous expression, enabling the discovery and modification of novel antimicrobials. The global antimicrobial resistance crisis demands new therapeutics to combat infectious pathogens. Bacteriocins, which can be broad or narrow spectrum, are promising tools for this purpose. However, few have progressed beyond preclinical trials. This review explores the diversity of bacteriocin structure and function, methods for identification and optimization, and the reasons for limited translation. Bacteriocins offer pharmacological advantages such as activity against pathogens in the nanomolar and picomolar ranges, narrow spectra, general non-toxicity, and distinct mechanisms of action. The diversity in bacteriocin structures and mechanisms also represents a range of ribosomally produced antimicrobials that are amenable to genetic engineering strategies enabling the production of bioengineered variants with superior efficacy against target microorganisms. Bacteriocins are natural agents of microbial competition, offering an advantage to bacteria over sensitive neighboring species, enabling invasion, colonization, and protection of a niche in complex environments such as the gut microbiome. By exploiting these natural mechanisms, they can be used as a tool for humans to deliver antimicrobial activity as pure peptides or composite nanoparticles or can be codified within live biotherapeutics for in situ expression in microbiomes. Advances in microbiome metagenomics and other 'omics' technologies have revealed catalogues of genome-mined and metabolome-mined bacteriocins, with new families continually being discovered. However, discovery is outpacing characterization and application. Narrow-target spectrum bacteriocins are increasingly desirable as we advance our understanding of host-microbiota interactions. Bacteriocins have been found to have a diverse set of other biological functions, including antiviral, cytotoxic, and immune-modulating activities. They can also modulate immune systems, promote plant growth, and support gene acquisition. Bacteriocins are typically cationic hydrophobic peptides, many of which are active at negatively charged bacterial cell surfaces. Their activities once they reach the cell surface are diverse, including specific interactions with cell-wall or cell-membrane components, general membrane interaction, pore formation, or receptor-mediated cell penetration and interference with intracellular processes. Nisin is well established to have a dual mechanism of action, binding the cell-wall precursor molecule lipid II, thereby inhibiting peptidoglycan biosynthesis and subsequent pore formation leading to cell death. Numerous post-translationally modified bacteriocins share a conserved nisin-like N-terminal lipid II binding domain but do not possess the same C-terminal domain and do not form pores. Many bacteriocins do not target