Krona is a web-based visualization tool for metagenomic data that allows interactive exploration of relative abundances and confidence levels within complex taxonomic or functional hierarchies. It uses a radial, space-filling display combined with parametric coloring and interactive polar-coordinate zooming. The tool is implemented in HTML5 and JavaScript, enabling fully interactive charts accessible via any modern web browser without the need for software installation. Each chart is an independent document, making it easy to share via email or a web server. Krona is compatible with popular metagenomic analysis tools and provides a rich, interactive interface that facilitates informed interpretations of metagenomic data. It is freely available under a BSD open-source license and can be used for a wide range of applications. Krona's architecture allows data to be stored locally while the interface is hosted online, enabling easy integration with existing websites. The tool supports various types of data inputs, including classifications from RDP, Phymm/PhymmBL, MG-RAST, and BLAST results. It also allows for the visualization of secondary attributes such as classification confidence. Krona's design addresses key tasks for interactive data exploration, including overview, zoom, filter, details-on-demand, relate, history, and extract. It provides a dynamic, interactive interface that supports zooming, filtering, and navigation through hierarchical data. Krona's visualization approach allows for the simultaneous display of abundance and hierarchy, with color gradients representing confidence levels. It is compared to other metagenomic visualization tools, such as MEGAN and MG-RAST, and is shown to provide more detailed and accurate representations of abundance and classification confidence. Krona's implementation is lightweight and modular, making it easy to integrate into existing analysis frameworks. It is also the first bioinformatics tool built entirely on HTML5, demonstrating the potential of web technologies for creating accessible and powerful visualization tools.Krona is a web-based visualization tool for metagenomic data that allows interactive exploration of relative abundances and confidence levels within complex taxonomic or functional hierarchies. It uses a radial, space-filling display combined with parametric coloring and interactive polar-coordinate zooming. The tool is implemented in HTML5 and JavaScript, enabling fully interactive charts accessible via any modern web browser without the need for software installation. Each chart is an independent document, making it easy to share via email or a web server. Krona is compatible with popular metagenomic analysis tools and provides a rich, interactive interface that facilitates informed interpretations of metagenomic data. It is freely available under a BSD open-source license and can be used for a wide range of applications. Krona's architecture allows data to be stored locally while the interface is hosted online, enabling easy integration with existing websites. The tool supports various types of data inputs, including classifications from RDP, Phymm/PhymmBL, MG-RAST, and BLAST results. It also allows for the visualization of secondary attributes such as classification confidence. Krona's design addresses key tasks for interactive data exploration, including overview, zoom, filter, details-on-demand, relate, history, and extract. It provides a dynamic, interactive interface that supports zooming, filtering, and navigation through hierarchical data. Krona's visualization approach allows for the simultaneous display of abundance and hierarchy, with color gradients representing confidence levels. It is compared to other metagenomic visualization tools, such as MEGAN and MG-RAST, and is shown to provide more detailed and accurate representations of abundance and classification confidence. Krona's implementation is lightweight and modular, making it easy to integrate into existing analysis frameworks. It is also the first bioinformatics tool built entirely on HTML5, demonstrating the potential of web technologies for creating accessible and powerful visualization tools.