Unison is a cross-platform tool for synchronizing collections of files over the internet. Unison has the somewhat unique ability to handle updates to multiple copies of a file. For many file formats, it is able to determine if the updates conflict with each other. If they do not, it can create a merged file incorporating changes from both updated versions. Otherwise, it will alert the user to the situation for them to resolve. Unison uses a bandwidth-efficient protocol similar to the well-known rsync program, so it is able to tolerate slow links (like public wifi). Unison is resilient to network failures. It will not corrupt files if synchronization is interrupted and synchronizations can be resumed later. Unison is free software licensed under the GPL with source code available on GitHub. Installers are available from the Unison website for Windows, macOS, and Linux.