Avogadro is an advanced chemical editor, visualization, and analysis platform designed for cross-platform use in computational chemistry, molecular modeling, bioinformatics, and related fields. It offers flexible, high-quality rendering and a powerful plugin architecture, enabling users to build molecular structures, format input files, and analyze computational chemistry outputs. Avogadro uses the CML file format as its native document type to enhance semantic accessibility of chemical data. The Avogadro library provides a framework with a code library and API, including 3D visualization capabilities, and has direct applications in chemistry, physics, materials science, and biology. The application features a rich graphical interface with dynamically loaded plugins, allowing users to extend functionality through C++ or Python plugins. Examples include a genetic algorithm for finding stable crystal structures and an interface with PackMol for creating solvated structures for molecular dynamics simulations. Avogadro is freely available under an open-source license and supports a wide range of features, including semantic chemical data editing, conversion, and integration with databases like PubChem and the Protein Data Bank. It can be extended via a powerful plugin mechanism to support new features in organic chemistry, inorganic complexes, drug design, materials, biomolecules, and simulations. The Avogadro project was initiated in 2007 to address the limitations of existing tools, offering a robust, flexible framework for molecular structure building and visualization. It uses modern programming languages and libraries, including C++, Qt, OpenGL, and Open Babel, and is licensed under the GNU GPLv2. Avogadro's architecture follows the model-view-controller paradigm, with plugins providing various functionalities such as color, display, tools, and extensions. It supports multiple display types, including ball-and-stick, Van der Waals, and surfaces, and offers tools for navigation, drawing, and adjusting molecular structures. The application also includes extensions for input generation, molecular property dialogs, and visualization of molecular orbitals and electron density. It supports Python scripting for plugin development, enabling easy prototyping and sharing. Avogadro has been widely adopted, with over 270,000 downloads and support for multiple languages. It is used in various scientific fields for building, editing, visualizing, and analyzing chemical and molecular data. The project continues to develop new features, including support for crystallography, nanoscience, and improved visualization techniques. Avogadro is freely available and welcomes new contributors in all areas.Avogadro is an advanced chemical editor, visualization, and analysis platform designed for cross-platform use in computational chemistry, molecular modeling, bioinformatics, and related fields. It offers flexible, high-quality rendering and a powerful plugin architecture, enabling users to build molecular structures, format input files, and analyze computational chemistry outputs. Avogadro uses the CML file format as its native document type to enhance semantic accessibility of chemical data. The Avogadro library provides a framework with a code library and API, including 3D visualization capabilities, and has direct applications in chemistry, physics, materials science, and biology. The application features a rich graphical interface with dynamically loaded plugins, allowing users to extend functionality through C++ or Python plugins. Examples include a genetic algorithm for finding stable crystal structures and an interface with PackMol for creating solvated structures for molecular dynamics simulations. Avogadro is freely available under an open-source license and supports a wide range of features, including semantic chemical data editing, conversion, and integration with databases like PubChem and the Protein Data Bank. It can be extended via a powerful plugin mechanism to support new features in organic chemistry, inorganic complexes, drug design, materials, biomolecules, and simulations. The Avogadro project was initiated in 2007 to address the limitations of existing tools, offering a robust, flexible framework for molecular structure building and visualization. It uses modern programming languages and libraries, including C++, Qt, OpenGL, and Open Babel, and is licensed under the GNU GPLv2. Avogadro's architecture follows the model-view-controller paradigm, with plugins providing various functionalities such as color, display, tools, and extensions. It supports multiple display types, including ball-and-stick, Van der Waals, and surfaces, and offers tools for navigation, drawing, and adjusting molecular structures. The application also includes extensions for input generation, molecular property dialogs, and visualization of molecular orbitals and electron density. It supports Python scripting for plugin development, enabling easy prototyping and sharing. Avogadro has been widely adopted, with over 270,000 downloads and support for multiple languages. It is used in various scientific fields for building, editing, visualizing, and analyzing chemical and molecular data. The project continues to develop new features, including support for crystallography, nanoscience, and improved visualization techniques. Avogadro is freely available and welcomes new contributors in all areas.