<p> This is a book about baseball players who performed in the major leagues before the age of 21. For years the dream of many boys has been to enter the world of professional sports. Out of millions of boys who have played baseball, more than 17,000 have appeared in major league contests. Among them were hundreds who made their debut before their 21st birthday. However, most of these appeared in only a few games. Only 284 young men have played at least one season as a regular before or during the season in which they reached the age of 21. They are the subjects of this book.</p><p> The text is divided into three parts. Part One deals with the careers of the ten prodigies who had the most productive seasons at the bat. Part Two discusses the ten young pitchers who had the most fruitful seasons on the mound. Part Three provides short sketches of the 172 players with at least five eligible seasons who do not rank among the above 20 prodigies. Data on the 92 players with at least one but fewer than five eligible seasons are given in an appendix.</p>