I describe reasons to think we are living in an eternally inflating
multiverse where the observable "constants" of nature vary from place to place.
The major obstacle to making predictions in this context is that we must
regulate the infinities of eternal inflation. I review a number of proposed
regulators, or measures. Recent work has ruled out a number of measures by
showing that they conflict with observation, and focused attention on a few
proposals. Further, several different measures have been shown to be
equivalent. I describe some of the many nontrivial tests these measures will
face as we learn more from theory, experiment, and observation.Comment: 20 pages, 3 figures; invited review for Classical and Quantum
Gravity; v2: references improve