Weasis is a free, open source DICOM viewer designed for both standalone and web-based use, featuring a highly modular architecture. It is widely utilized in healthcare settings, including hospitals, health networks, multicenter research trials, and by patients. As cross-platform software, Weasis offers flexible integration with PACS, RIS, HIS, or EHR systems. The viewer leverages the OpenCV library to deliver high-performance and high-quality medical imaging renderings. From version 4 onwards, Weasis features a responsive user interface aligned with operating system options, offering an enhanced experience on high-resolution screens. Key features include support for a wide range of DICOM files, such as multi-frame, enhanced, MPEG-2, MPEG-4, and more. Users can import DICOM files via DICOM Query/Retrieve (C-GET, C-MOVE, and WADO-URI) and DICOMWeb (QUERY and RETRIEVE), as well as import and export DICOM CD/DVD with DICOMDIR.