Baseball analysts often criticize pitcher win-loss records as a poor measure of pitcher performance, as wins are the product of team performance. Fans criticize WAR (Wins Above Replacement) because it takes in theoretical rather than actual wins. Player won-lost records bridge the gap between these two schools of thought, giving credit to all players for what they do--without credit or blame for teammates' performance--and measuring contributions to actual team wins and losses. The result is a statistic of player value that quantifies all aspects of individual performance, allowing for robust comparisons between players across different positions and different seasons. Using play-by-play data, this book examines players' won-lost records in Major League Baseball from 1930 through 2015.