My suggestion: Reviewable plays are force plays at a base and fair/foul calls. At this time, until replay can be better evaluated, I would not implement review of tag plays or catch/trap plays, which adds some difficulty to the situation. Reviewing force plays will eliminate a very high amount of questionable calls. A 5th umpire is assigned to the crew as the "replay" umpire and reviews all such close plays immediately in the booth after the conclusion of the play. He will have until the first pitch of the next AB, which will give him about 30-60 seconds to look over the play. If he determines that an overturn is necessary, he will "buzz" down to the homeplate umpire. Fair/foul runner placement: This one is easy. A fair changed to a foul is a strike on the hitter, runners return to their respective bases and batter returns to the batters box. A foul changed to a fair is ruled a ground rule double, and the runners advance two bases. An exception is if the fair ball call is ruled in the infield, in which case one base will be awarded for the hitter and every runner. Force play runner placement: If a safe is changed to an out, and it is the 3rd out, the inning is over with no runs scored just as if it occurred. If it is the 2nd or 1st out, the out is recorded and all other continuous action as part of the play counts, except for anything that the "out" runner did. An "out" changed to a "safe" would be one of the most difficult. This is where things could get very complex. (Example: Batter hits a groundball to 3rd base with a runner on 1st. Throw to 2nd is incorrectly called "out". Relay throw goes over the first baseman's head and the batter winds up at 2nd where the original runner was first incorrectly called out). The only solution I can come up with is to count any outs, base advancements, or runs that occur as a result of continuous action, and then place the runners at the discretion of the umpire. You could also call the play dead at the spot of the wrong call, with the runners awarded the base they were en route to, but you'll eventually see a hitting team get screwed by this. (Tie game in the 9th, man on 2nd. Ground ball to left side, hitter called out at 1st, other runner on his way to 3rd. Throw to third goes into foul territory and winning run comes home to score. Then a review comes and calls the runner "safe" at 1st and replaces the runners). Only solution I can think of to this is to allow the team "benefiting" from the changed call the option of declining the review result and accepting the play as is. There are some cases where there is not an easy solution. I'm definitely pro-replay, but the more I talk about it the more I understand the hesitations, because there are so many different possible scenarios to account for that make it very difficult.