The article provides an overview of the diversity, classification, and evolution of CRISPR-Cas systems in bacteria and archaea. CRISPR-Cas systems are adaptive immune systems that use RNA-guided nucleases to defend against foreign genetic material. These systems are classified into two main classes: Class 1, which has multi-subunit effector complexes, and Class 2, which has single-protein effector modules. Recent studies have identified new types and subtypes of Class 2 CRISPR-Cas systems, including type VI systems, which are the first to exclusively target RNA. Comparative analysis suggests that Class 2 systems evolved from mobile genetic elements multiple times. The article also discusses the functional and structural diversity of CRISPR-Cas systems, such as the RNA-targeting activity of type VI systems, pre-crRNA processing by Class 2 effectors, and the regulation of type VI-B effectors by small Cas proteins. The evolutionary origins of CRISPR-Cas systems are explored, highlighting the contributions of mobile genetic elements like Casposons and transposons. The article concludes by discussing the recent advances in understanding the diversity and evolution of CRISPR-Cas systems and their potential applications in genome editing and regulation.The article provides an overview of the diversity, classification, and evolution of CRISPR-Cas systems in bacteria and archaea. CRISPR-Cas systems are adaptive immune systems that use RNA-guided nucleases to defend against foreign genetic material. These systems are classified into two main classes: Class 1, which has multi-subunit effector complexes, and Class 2, which has single-protein effector modules. Recent studies have identified new types and subtypes of Class 2 CRISPR-Cas systems, including type VI systems, which are the first to exclusively target RNA. Comparative analysis suggests that Class 2 systems evolved from mobile genetic elements multiple times. The article also discusses the functional and structural diversity of CRISPR-Cas systems, such as the RNA-targeting activity of type VI systems, pre-crRNA processing by Class 2 effectors, and the regulation of type VI-B effectors by small Cas proteins. The evolutionary origins of CRISPR-Cas systems are explored, highlighting the contributions of mobile genetic elements like Casposons and transposons. The article concludes by discussing the recent advances in understanding the diversity and evolution of CRISPR-Cas systems and their potential applications in genome editing and regulation.