A bonus baby was an amateur player who signed for a large enough contract that the team was required to keep him on the major league roster for a full season (or two) instead of playing him in the minor leagues. The so-called bonus rule was designed to limit the dominance of teams with deep farm systems and lots of money. The rule was in effect from 1947 through 1965. Harmon Killebrew and Sandy Koufax went on to the greatest career by a bonus baby, but many of the players did not fare well as their development was hindered by sitting on the bench when they should have been playing regularly.
See List of baseball players who went directly to the major leagues for a list of "bonus babies".
- Brent Kelley: Baseball's Bonus Babies: Conversations with 24 High-Priced Ballplayers Signed from 1953 to 1957, McFarland, Jefferson, NC, 2006.
- Steve Treder: "Cash in the Cradle: The Bonus Babies", The Hardball Times, May 25, 2006. 
- Sam Zygner: "Phillies Bonus Babies, 1953–57", in Morris Levin, ed.: From Swampoodle to South Philly: Baseball in Philadelphia and the Delaware Valley, The National Pastime, SABR, 2013, pp. 92-97.