The CRISPR-Cas systems are adaptive immune systems found in many archaea and bacteria, used to defend against invading genetic elements such as viruses and plasmids. These systems are encoded by operons with diverse architectures and high evolutionary rates. The authors propose a 'polythetic' classification based on multiple criteria, including the phylogenies of cas genes, CRISPR repeat sequences, and locus architecture. Three major types of CRISPR-Cas systems are identified, with further subtypes and chimeric variants. The systems function through three stages: adaptation, expression, and interference. During adaptation, short DNA fragments are integrated into the CRISPR locus. Expression involves processing the CRISPR locus into small RNAs, while interference targets and cleaves foreign DNA or RNA. The classification of CRISPR-Cas systems is complex due to their dynamic evolution and genomic diversity. The authors suggest a unified classification that integrates phylogenetic and genomic data. The existing classification is inadequate due to inconsistencies in nomenclature and the complexity of evolutionary relationships. A new classification is proposed that groups systems based on cas1 and cas2 genes, with subtypes defined by signature genes. The three main types (I, II, III) have distinct characteristics and distributions among archaea and bacteria. Type II systems are more common in bacteria, while type III systems are more prevalent in archaea. The classification includes subtypes with specific signature genes and addresses challenges in naming and classification due to gene fusions and recombination. The authors emphasize the need for a flexible, unified classification that incorporates phylogenetic, structural, and genomic data to accurately reflect the diversity and complexity of CRISPR-Cas systems.The CRISPR-Cas systems are adaptive immune systems found in many archaea and bacteria, used to defend against invading genetic elements such as viruses and plasmids. These systems are encoded by operons with diverse architectures and high evolutionary rates. The authors propose a 'polythetic' classification based on multiple criteria, including the phylogenies of cas genes, CRISPR repeat sequences, and locus architecture. Three major types of CRISPR-Cas systems are identified, with further subtypes and chimeric variants. The systems function through three stages: adaptation, expression, and interference. During adaptation, short DNA fragments are integrated into the CRISPR locus. Expression involves processing the CRISPR locus into small RNAs, while interference targets and cleaves foreign DNA or RNA. The classification of CRISPR-Cas systems is complex due to their dynamic evolution and genomic diversity. The authors suggest a unified classification that integrates phylogenetic and genomic data. The existing classification is inadequate due to inconsistencies in nomenclature and the complexity of evolutionary relationships. A new classification is proposed that groups systems based on cas1 and cas2 genes, with subtypes defined by signature genes. The three main types (I, II, III) have distinct characteristics and distributions among archaea and bacteria. Type II systems are more common in bacteria, while type III systems are more prevalent in archaea. The classification includes subtypes with specific signature genes and addresses challenges in naming and classification due to gene fusions and recombination. The authors emphasize the need for a flexible, unified classification that incorporates phylogenetic, structural, and genomic data to accurately reflect the diversity and complexity of CRISPR-Cas systems.