Igor explained RF well enough, although they use defensive innings / 9 as the divisor. And those numbers can be skewed by the number of chances a player gets. A flyball heavy staff hurts infielders' ratings. A K-heavy staff hurts everybody's ratings. Etc. And, of course, there are park effects to consider. I'm sure having an infield like a hayfield helps. ZR is like Igor said, basically plays made per chances. There are again limits to the accuracy of ZR. Ball's that hit 20 feet up on the Green Monster are technically counted as "in the zone", high walls in general are going to hurt a players rating, and the size of an outfielder's zone can change drastically from park to park. Centerfield in Miller Park isn't the same as centerfield in Pro Player stadium. And more foul territory, like in Oakland, can only help a corner player's rating. And there are limits. ZR doesn't do a good job, IMO, on gauging a player's ability to turn the DP. Or a first baseman's ability to receive the ball. Or a catcher's ability to block balls in the dirt and throw out runners. And they've periodically changed the zones, which makes career ZR a misleading statistic when comparing it to a specific year. One of the things that I do like about ZR and RF is that they do take the scorekeepers, often faulty, judgement out of the statistic. Wes Helms being a good example. Those plays could have been ruled differently by different scorekeepers, but they're still counted the same in ZR and RF. I generally like using the whole set of F%, RF, and ZR when evaluating a player. I think they're simply more descriptive. They help identify the sure handed statues. Or the error prone players that can make up for it via range and arm strength. And the descriptive nature adds value. Robert