Rule 5 draft
The Rule 5 draft is a Major League Baseball player draft that occurs each year in December, at the annual Winter Meeting of general managers. The Rule 5 draft aims to prevent teams from stockpiling too many young players on their minor league affiliate teams when other teams would be willing to have them play in the majors. The Rule 5 draft is named for its place in Major League Rules. (It is sometimes mistakenly referenced with a Roman numeral.) The June Rule 4 draft, known as simply "the draft", "amateur draft", or "first year player draft", is a distinctly different process in which teams select high school and college players.
Prior to its current incarnation, from 1959 until the creation of the Rule 4 draft in 1965, top first-year prospects of teams were exposed to the Rule 5 draft. This was in some ways an outgrowth of the Bonus Rule that existed in 1946–50 and 1953–57.
As in the amateur draft, the selection order of the teams is based on each team's win-loss record from the prior regular season, each round starting with the team with the worst record and proceeding in order to the team with the best record. Any player selected under Rule 5 is immediately added to his new team's 40-man roster; thus, teams who do not have an available roster spot may not participate in the Rule 5 draft. Players who are not currently on their team's 40-man roster are eligible to be selected in the Rule 5 draft, but only after a standard exemption period has elapsed. See Selection eligibility below.
If chosen in the Rule 5 draft, a player must be kept on the selecting team's 25-man major league roster for the entire season after the draft—he may not be optioned or designated to the minors. The selecting team may, at any time, waive the Rule 5 draftee. If a Rule 5 draftee clears waivers by not signing with a new MLB team, he must be offered back to the original team, effectively canceling the Rule 5 draft choice. Once a Rule 5 draftee spends an entire season on his new team's 25-man roster, his status reverts to normal and he may be optioned or designated for assignment.
To prevent the abuse of the Rule 5 draft, the rule also states that the draftee must be active for at least 90 days. This keeps teams from drafting players, then placing them on the disabled list for the majority of the season. For example, if a Rule 5 draftee was only active for 67 days in his first season with his new club, he must be active for an additional 23 days in his second season to satisfy the Rule 5 requirements.
Any player chosen in the Rule 5 draft may be traded to any team while under the Rule 5 restrictions, but the restrictions transfer to the new team. If the new team does not want to keep the player on its 25-man roster for the season, he must be offered back to the team of which he was a member when chosen in the draft.
Selection eligibility 
Players are eligible for selection in the Rule 5 draft who are not on their major league organization's 40-man roster and:
– were 18 or younger on the June 5 preceding their signing and this is the fifth Rule 5 draft upcoming; or
– were 19 or older on the June 5 preceding their signing and this is fourth Rule 5 draft upcoming.
These exemption periods (one year longer than those in effect previously) went into effect as part of a new Collective Bargaining Agreement in October 2006. The change took effect immediately, exempting many players from the 2006 Rule 5 draft even though they had been signed in some cases more than four years before the new agreement came into effect. Prior to the rule change, players were exempt from the first two or three Rule 5 drafts held after their signing (regardless of the year they were drafted), rather than from the first three or four Rule 5 drafts after their signing.
Cost and example 
To prevent excessive turnover in the minor league levels, each draftee costs $50,000. If the draftee does not stay on the selecting team's 25-man (major league) roster all season, the player must be offered back to his original team at half-price. Organizations may also draft players from AA or lower to play for their AAA affiliates (for $12,000) and may draft players from A teams or lower to play for their AA affiliates (for $4,000).
The Rule 5 draft has opened opportunities for teams to take other teams' top prospects who may not be ready for the major leagues. A prominent recent example is Johan Santana, who was chosen in the 1999 Rule 5 draft by the Florida Marlins when the Houston Astros declined to put him on their 40-man roster. The Marlins traded Santana to the Minnesota Twins in a pre-arranged deal for cash and Jared Camp who was taken in that same rule 5 draft by the Twins immediately before Florida chose Santana.
The Twins kept Santana on their roster for the 2000 season, despite the pitcher's subpar performance that season (6.49 earned run average) which was unsurprising given his youth and inexperience. After the 2000 season, the Twins had the right to option Santana to their minor league system, but chose not to in 2001. He was briefly optioned to AAA at the start of the 2002 season. He returned that year to the major leagues and established himself as an above-average pitcher. Since 2004, Santana has won two Cy Young awards. Santana had not played above low A ball before being chosen in the Rule 5 draft, and he likely would not have made his major league debut until at least the 2001 or the 2002 season with the Astros, if at all.
Notable Rule 5 draftees 
Hall of Famers 
Other notable selections 
Drafted, but returned or traded before start of season 
Drafted, then traded to the drafting team, nullifying the draft 
Schwarz, Alan (2002). "Ask BA". Baseball America. Retrieved 2006-04-19.