The article introduces KAAS (KEGG Automatic Annotation Server), a web-based server designed to automate the identification of functional properties and biological roles of genes in genomes. KAAS uses sequence similarities, bi-directional best hit (BBH) information, and heuristics to assign K numbers (KEGG Orthology identifiers) to genes, enabling the reconstruction of KEGG pathways and BRITE hierarchies. The method is based on the KEGG GENES database, which provides a standardized mechanism for cross-species annotation. KAAS can handle both amino acid and nucleotide sequences and offers three output views: KO list, KO hierarchy, and pathway map. The server's accuracy is tested against manually curated KEGG GENES data, showing high sensitivity and specificity for various organisms. The article also discusses the computational time and the impact of different reference data sets on annotation accuracy.The article introduces KAAS (KEGG Automatic Annotation Server), a web-based server designed to automate the identification of functional properties and biological roles of genes in genomes. KAAS uses sequence similarities, bi-directional best hit (BBH) information, and heuristics to assign K numbers (KEGG Orthology identifiers) to genes, enabling the reconstruction of KEGG pathways and BRITE hierarchies. The method is based on the KEGG GENES database, which provides a standardized mechanism for cross-species annotation. KAAS can handle both amino acid and nucleotide sequences and offers three output views: KO list, KO hierarchy, and pathway map. The server's accuracy is tested against manually curated KEGG GENES data, showing high sensitivity and specificity for various organisms. The article also discusses the computational time and the impact of different reference data sets on annotation accuracy.