Wikipedia has a great in-depth write-up (http://en.wikipedia.org/wiki/Major_League_Baseball_transactions), but here are the highlights: - From the end of the previous World Series through July, any player under contract may be traded - After 31 Jul, players must either 1)clear waivers or 2) not be on any team's 40-man roster - If a player is acquired after 31 Aug, they cannot be on a postseason roster unless replacing an injured player No-Trade clauses: - 10/5 rule: if a player has been in the majors 10 years and on the same team for 5 years, they must consent to the trade - A few high-profile players not meeting the 10/5 rule have no-trade clauses negotiated in their contract Waivers: When a player is placed on waivers, any team interested may claim him within 3 business days. If multiple teams claim him, the team with the worst record in the same league gets preference, and the original team must trade the player to them within 2 days, rescind the waiver or do nothing and allow the claiming team to assume the player's contract. If no team claims the player in 3 business days, they have "cleared waivers". The original team may then freely trade the player, assign him to the minors, or release him.