Genomic islands (GEIs) are discrete DNA segments that facilitate horizontal gene transfer (HGT) and contribute to bacterial genome plasticity and evolution. GEIs can be mobile or non-mobile, and they vary in size and content among closely related strains. They play a crucial role in the dissemination of antibiotic resistance, virulence genes, and catabolic pathways, leading to the emergence of "superbugs" and new metabolic capabilities. GEIs are often inserted at tRNA genes, flanked by direct repeats, and contain genes encoding integrases, transposases, and other elements involved in their transfer and excision. The evolutionary origins of GEIs are diverse, including phage-related and conjugative plasmid-derived elements. The transfer of GEIs between bacteria can occur through transformation, conjugation, and transduction, influenced by host background and environmental conditions. Regulation of GEI transfer is complex and influenced by various factors, including stress responses and quorum sensing. GEIs contribute to the evolution of pathogenic bacteria, such as methicillin-resistant *Staphylococcus aureus* (MRSA) and virulent *Enterococcus faecalis*, by facilitating the acquisition of genes for antibiotic resistance, virulence, and other adaptive traits.Genomic islands (GEIs) are discrete DNA segments that facilitate horizontal gene transfer (HGT) and contribute to bacterial genome plasticity and evolution. GEIs can be mobile or non-mobile, and they vary in size and content among closely related strains. They play a crucial role in the dissemination of antibiotic resistance, virulence genes, and catabolic pathways, leading to the emergence of "superbugs" and new metabolic capabilities. GEIs are often inserted at tRNA genes, flanked by direct repeats, and contain genes encoding integrases, transposases, and other elements involved in their transfer and excision. The evolutionary origins of GEIs are diverse, including phage-related and conjugative plasmid-derived elements. The transfer of GEIs between bacteria can occur through transformation, conjugation, and transduction, influenced by host background and environmental conditions. Regulation of GEI transfer is complex and influenced by various factors, including stress responses and quorum sensing. GEIs contribute to the evolution of pathogenic bacteria, such as methicillin-resistant *Staphylococcus aureus* (MRSA) and virulent *Enterococcus faecalis*, by facilitating the acquisition of genes for antibiotic resistance, virulence, and other adaptive traits.