Pannellum is a lightweight, free, and open source panorama viewer for the web. Built using HTML5, CSS3, JavaScript, and WebGL, it provides plugin-free display of panoramic images in equirectangular, cubemap, and multiresolution formats. The viewer supports hot spots for linking scenes or displaying information, enabling virtual tours. It includes a comprehensive API for programmatic control, device orientation support for mobile devices, fullscreen capability, and responsive touch and mouse controls. The architecture separates core WebGL rendering logic (in libpannellum.js) from viewer UI and API logic (in pannellum.js), with fallback to CSS 3D transforms when WebGL is unavailable. A utility script (generate.py) creates multiresolution tile sets for efficient streaming of large panoramas.