Yesterday, Microsoft officially unveiled its Fluent Design System at its Build developers conference. It was previously known as Project Neon.
The Fluent Design System is designed to be the true successor to Microsoft’s Metro design, and will appear across apps and services on Windows, iOS, and Android.
Microsoft is planning to implement these subtle design changes gradually. Some are already available in new updates to existing Windows 10 apps, and more will start to appear in Windows itself as Microsoft updates the operating system with the Fall Creators Update and future updates. “It’s going to be a journey,” says Microsoft director Aaron Woodman, noting that these design changes will appear over time in Windows and other products. 
In his presentation at Build yesterday, Microsoft’s Joe Belfiore demonstrated a number of Fluent Design changes. “You’re going to see Fluent Design show up in the Windows shell, in our apps, and across devices,” explains Joe Belfiore. Microsoft is focusing on light, depth, motion, material, and scale for its Fluent Design, with subtle changes that make the design feel like it’s moving during interactions in Windows.
 Tom Warren, Fluent Design is Microsoft’s new Metro UI for Windows, The Verge, May 11, 2017, https://www.theverge.com/2017/5/11/15615812/microsoft-fluent-design-system-project-neon-features.