Epic has made the first Preview build of Unreal Engine 4.18 available through the Epic Games launcher. Developers can now experiment with volumetric lightmaps, enhanced static skylight directionality, major clothing tool upgrades, the revamped Physics Asset Editor and Media Framework 3.0. Improvements have also been made to support for both Google's ARCore and Apple's ARKit. The desktop forward renderer on iOS, used in the Wingnut AR demo at WWDC 2017, is now available as well.
A full list is available on the UE forums.