The Hurd currently runs only on the Mach microkernel, but there is work to port it to the newer L4 microkernel. This article has the details about the current status of the Hurd/L4 port. It is primarily written for people familiar with Hurd/Mach, but who do not know L4.

Because of design differences between L4 and Mach (as is to be expected from 2 different kernels), a lot of infrastructural work still needs to be done; including a Virtual Memory Manager (VMM), getting glibc ported, and writing device drivers.