.msi File — Windows Installer Package

MSI (Microsoft Installer) is the Windows Installer package format used for installing, maintaining, and removing software on Windows systems. MSI files are relational databases (using the COM Structured Storage format) containing installation instructions, file data, registry entries, shortcuts, environment variables, and custom action scripts organized into predefined tables. The Windows Installer service (msiexec.exe) processes MSI packages, providing standardized installation with rollback support, repair capabilities, silent installation for enterprise deployment, and clean uninstallation that removes all installed components. MSI packages support features (optional components), transforms (.mst files for customizing installations), patches (.msp files for updates), and merge modules (.msm for shared components). Group Policy can deploy MSI packages across enterprise networks. MSI files can be created using WiX Toolset (open source), InstallShield, Advanced Installer, or Visual Studio. While EXE installers offer more flexibility for custom UIs and complex logic, MSI is preferred in enterprise environments for its standardized behavior, remote deployment support, and predictable uninstall behavior. The format supports digital signatures for publisher verification.