The ARB (from Latin arbor, tree) project, initiated nearly 10 years ago, is a comprehensive software environment for sequence data management and analysis. It comprises a variety of interacting tools controlled by a common graphical user interface, initially designed for ribosomal RNA data but applicable to any nucleic and amino acid sequence data. The central database stores processed primary structure data and additional descriptive information, which can be linked via local or worldwide networks. Key features include data import and export, sequence alignment, primary and secondary structure editing, profile and filter calculation, phylogenetic analyses, and probe design and evaluation. The package is widely used by numerous research groups globally. The software is developed for UNIX systems, primarily using C++ and C, with some parts in Perl and other script languages. It integrates functionalities from the GDE project and PHYLIP package, among others. The ARB package supports various data visualization methods, sequence editing modes, and advanced tools for phylogenetic treeing and probe design. Future developments focus on web-based probe target site prediction and genome analysis capabilities.The ARB (from Latin arbor, tree) project, initiated nearly 10 years ago, is a comprehensive software environment for sequence data management and analysis. It comprises a variety of interacting tools controlled by a common graphical user interface, initially designed for ribosomal RNA data but applicable to any nucleic and amino acid sequence data. The central database stores processed primary structure data and additional descriptive information, which can be linked via local or worldwide networks. Key features include data import and export, sequence alignment, primary and secondary structure editing, profile and filter calculation, phylogenetic analyses, and probe design and evaluation. The package is widely used by numerous research groups globally. The software is developed for UNIX systems, primarily using C++ and C, with some parts in Perl and other script languages. It integrates functionalities from the GDE project and PHYLIP package, among others. The ARB package supports various data visualization methods, sequence editing modes, and advanced tools for phylogenetic treeing and probe design. Future developments focus on web-based probe target site prediction and genome analysis capabilities.