List of Major League Baseball progressive single-season home run leaders

The Major League Baseball single-season record for the number of home runs hit by a batter has changed many times over the years.

Summarized from The Baseball Encyclopedia for older information, and from various news stories for newer information:[vague] Number of home runs in the season divided by number of games in schedule (not games played)

Four baseball bats that were used in setting single-season home run records. From left to right: bat used by Babe Ruth to hit his 60th home run during the 1927 season, bat used by Roger Maris to hit his 61st home run during the 1961 season, bat used by Mark McGwire to hit his 70th home run during the 1998 season, and the bat used by Sammy Sosa for his 66th home run during the same season.