Cygwin是許多自由軟體的集合，最初由Cygnus Solutions開發，用於各種版本的Microsoft Windows上，執行UNIX類系統。Cygwin的主要目的是透過重新編譯，將POSIX系統（例如Linux、BSD，以及其他Unix系統）上的軟體移植到Windows上。Cygwin移植工作在Windows NT、Windows 2000、Windows XP以及Windows Server 2003上比較好，在Windows 95和Windows 98上，相對差勁一些。目前Cygwin由Red Hat等負責維護。

When installing packages for the first time, setup.exe does not install every package. Only the minimal base packages from the Cygwin distribution are installed by default. Clicking on categories and packages in the setup.exe package installation screen will provide you with the ability to control what is installed or updated. Clicking on the "Default" field next to the "All" category will provide you with the opportunity to install every Cygwin package. Be advised that this will download and install hundreds of megabytes to your computer. The best plan is probably to click on individual categories and install either entire categories or packages from the categories themselves.