Application modernisation
MSI
Microsoft Installer package
MSI, the Microsoft Installer package format, is the traditional Windows installer technology built around the Windows Installer service. An .msi file is not a simple archive but a relational database of tables describing every action the installation needs to perform: which files to copy and where, which registry keys to create, and which shortcuts to add.
Why MSI matters in a Microsoft estate
MSI matters during Windows 11, Intune, Azure Virtual Desktop, and Cloud PC programmes because application blockers can delay the whole rollout. The practical question is whether the term helps capture, package, sign, deploy, or troubleshoot an app with less rework.
How MSI shows up in practice
In many real-world packages it also specifies which custom actions to run, arbitrary scripts or executables invoked during install that fall outside the standard, well-behaved Windows Installer actions the format was designed around. MSI has been the dominant enterprise application packaging format for decades and remains widely deployed today, particularly for applications that genuinely need machine-wide installation, elevated system access, driver installation, or Windows service registration. None of these map cleanly onto MSIX's per-package isolation model. This is why MSI and scripted alternatives like PSADT continue to have a legitimate place alongside MSIX in a mixed packaging estate, rather than being fully replaceable by it. The characteristic weakness that has driven much of the industry's move toward MSIX is MSI's uninstall reliability.
An MSI's custom actions can write files and registry keys outside what the Windows Installer engine itself tracks, and years of install, patch, and uninstall cycles accumulate small inconsistencies. As a result, uninstalling an MSI-packaged application frequently leaves behind orphaned registry keys, residual files, and configuration drift that a clean uninstall should have removed, a problem that compounds across a device's lifetime as more applications are installed and removed. In a packaging and estate management context, MSI is most often encountered from two directions. The first is as the source material for converting a legacy application into MSIX, where having the original MSI available is generally the cleanest starting point because its declarative table structure can be translated fairly directly, in contrast to capturing a live installation when the original installer has been lost.
The second is as the target format itself for applications a packaging team has deliberately decided not to convert, because the effort or risk of resolving MSIX's isolation constraints outweighs the benefit for that specific application. Licence reclaim and cost-optimisation tooling also intersects with MSI in practice. Many older, high-value line-of-business applications an organisation is trying to track usage of or right-size licensing for are still MSI-packaged, so estate visibility work frequently has to reconcile MSI-based deployment records with the application and licence inventory a cost or compliance team is trying to maintain.