The deadline is 4 p.m. ET on Thursday, July 31. Until then teams may trade players freely without having to go through the waiver process.
What happens after the deadline? And what are waivers?
After the July 31 deadline, any player on a 40-man roster must clear Major League waivers before being traded. That is, the player must be offered to the other teams in reverse order of the standings, and if he is claimed by one of the teams he cannot be traded. The club that made the waiver request can either withdraw the request and keep the player or let the player go to the claiming team, which would then have the rights to the player and be obligated to that player's current contract.
A waiver, which is a permission granted for certain assignments of player contracts, can get complicated if more than one team makes a waiver claim. If more than one club in the same league makes a claim, then the club currently lower in the standings gets the player. If clubs in both leagues claim the player, preference goes to the club in the same league as the club requesting waivers.
Do teams often put in waiver claims simply to "block" a trade?
Once in a while it happens, but not very often. There is a risk involved, especially if the player trying to go through waivers has a large contract. By claiming a player, a team could prevent that player from being traded to a division rival. Of course, if a team claims a player and that player isn't pulled back by the team that requested waivers, the claiming team could get that player, contract and all.
An example of that happened in 1998, when Toronto was attempting to deal former All-Star closer Randy Myers late in the year to Atlanta. The Padres, in the midst of a pennant race, put in a waiver claim for the veteran left-hander to block him from being traded to the Braves. The Blue Jays let Myers go to the Padres rather than pull him back from the waiver process. Myers appeared in just 14 1/3 innings for the Padres, going 1-3 with no saves, and he did not pitch after the '98 season, leaving San Diego on the hook for the balance of his $13.6 million salary for 1999-2000.
So there is no final deadline for trades?
No. But the real deadline for summer trades may be Aug. 31 at midnight ET. After that, any player added to a Major League roster will not be eligible for the postseason.