This repository is Microsoft's Fluent UI design system implementation for Apple platforms (iOS, macOS, visionOS). It provides a comprehensive library of native UI controls (buttons, calendars, drawers, tooltips, navigation bars, etc.) built on UIKit, AppKit, and SwiftUI, all unified under a token-based theming system. The architecture is a cross-platform library with shared common code (FluentUI_common) and platform-specific implementations (FluentUI_iOS, FluentUI_macOS), plus demo applications and integration tests for each platform. The core innovation is a design token system where every component declares a ControlTokenSet that maps visual properties (colors, fonts, shadows, spacing) to theme-aware values. This allows consistent theming across all controls, with overrides at the theme-wide, per-control, and per-instance level. The library also includes a SwiftUI/UIKit bridge (ControlHostingView) for embedding SwiftUI components in UIKit view hierarchies, enabling gradual migration and cross-platform code reuse. The repository is actively maintained with CI/CD pipelines, localization support, and extensive UI automation tests. While the codebase is well-structured and demonstrates strong engineering practices, it shows signs of platform fragmentation (separate iOS/macOS implementations for some components) and relies on conditional compilation for version-specific features, which increases maintenance complexity. Overall, it is a production-grade UI framework suitable for building Microsoft 365 experiences on Apple devices.