Windows 11 still relies on code from the 90s as its bedrock, says Microsoft's CTO

At a glance:

• Microsoft CTO Mark Russinovich confirms Win32 code from the 1990s remains the bedrock of Windows 11 in 2026.
• When asked if 90s engineers expected Win32 to still be a first-class API surface in 2026, Russinovich said they were dreaming of flying cars and moon stations instead.
• Even Microsoft's own WinRT project failed to replace Win32, and the CTO has no expectation that his original functions would become more relevant than ever three decades later.

The 90s code that won't die

Windows 11 is, in many ways, a paradox. It ships with sleek new features and a redesigned interface, yet peel back a layer and you will find yourself staring at Control Panel — a relic that feels like a time warp to the Windows 95 era. Microsoft's own CTO, Mark Russinovich, has now publicly confirmed what many power users have long suspected: a significant portion of the operating system's codebase traces back to the 1990s, and it shows no sign of going anywhere.

In a video posted to X by Microsoft Dev Docs, Russinovich was asked a pointed question: did the engineers who wrote Win32 in the 1990s ever imagine that their API would still be a first-class surface in 2026? His answer was a candid no. According to Russinovich, the team was more preoccupied with visions of flying cars and moon stations by the year 2026 than with the longevity of the code they were writing. He did not anticipate that the functions he authored decades ago would become "more relevant than ever" in the modern era.

Why Win32 became unkillable

The reason Win32 persists is not a lack of trying. Microsoft itself attempted to reboot the API surface with WinRT, a project designed to offer a fresh, modern alternative to the aging Win32 foundation. But as Russinovich explains, WinRT ended up becoming its own thing entirely rather than supplanting the older layer. The problem is one of sheer entrenchment: thousands of applications, frameworks, and developer tools were built on top of Win32 over the past three decades. That collective weight has turned Win32 into the bedrock of the operating system.

Once an ecosystem reaches that level of dependency, ripping out the foundation becomes virtually impossible without breaking everything that sits on top. Russinovich acknowledges this reality directly — the Win32 base is simply too important to replace. Every new Windows release, including Windows 11, must maintain compatibility with that legacy layer to avoid alienating the enormous install base of existing software.

What this means for Windows 11 users

For the average Windows 11 user, the Win32 underpinning is largely invisible. The polished shell, the new Settings app, and modern features like Snap Layouts and virtual desktops give the impression of a thoroughly contemporary OS. But the moment you navigate to Control Panel — still accessible in Windows 11 — the decades-old UI aesthetic reasserts itself. It is a reminder that beneath the surface-level modernization, the operating system is carrying decades of architectural debt.

This is not necessarily a flaw in the way most users experience Windows. The abstraction layer is thick enough that day-to-day tasks, web browsing, office productivity, and gaming run without anyone needing to think about the age of the underlying API calls. However, for developers and IT administrators who interact with the Win32 surface directly, the persistence of 1990s-era code can mean dealing with quirks, deprecated patterns, and security considerations that date back to an era when the threat landscape looked very different from today.

The broader lesson on software longevity

Russinovich's remarks echo a broader truth in software engineering: code that survives is code that people depend on. The 1990s were a formative decade for Windows, and the decisions made during that period — around memory management, windowing, process handling, and system calls — created an API contract so widely adopted that it effectively became infrastructure. No single company, not even Microsoft, can easily retire infrastructure that an entire industry has built upon.

The interview also serves as a humbling reminder for engineers. When Russinovich wrote those early functions, he had no expectation they would remain relevant for thirty years. The unpredictability of software longevity is one of the few constants in a field that otherwise prizes novelty and iteration. For Microsoft, the challenge going forward is not just adding new features to Windows 11 but managing a codebase that is older than many of the developers now maintaining it.

What to watch next

As Windows 11 continues to receive updates through 2026 and beyond, the tension between legacy compatibility and modern architecture will only grow. Microsoft has been gradually introducing new APIs and runtime environments, but the Win32 bedrock shows no sign of being retired. Developers building for Windows today should expect Win32 to remain a first-class surface for the foreseeable future, and the Control Panel ghost will likely haunt the OS for years to come.

The full interview is available via the Microsoft Dev Docs X account and is worth watching for anyone curious about the internal tensions that come with maintaining one of the world's most widely used operating systems.