The magnetorotational instability (MRI) is a mechanism whereby the inclusion of a magnetic field may destabilize an otherwise hydrodynamically stable differential rotation. It is believed to play an important role in the dynamics of astrophysical accretion disks. There is therefore considerable interest in obtaining the MRI in the lab as well. I will present theoretical calculations showing that it is much more easily excited in a helical rather than a purely axial field, and will then describe an experiment that has succeeded in obtaining this so-called helical MRI in a liquid metal Taylor-Couette apparatus.