Cleanup hitters in baseball have always been judged on their ability to drive in runs. For decades, that's been their main responsibility.
The third spot in the batting order is generally reserved for each team's best hitter overall, combining a solid batting average, on-base percentage and power. But cleanup hitters make their living by plating as many baserunners as possible.
George Bell is a great example—throughout his career he struck out over two times as much as he walked, but he was his team's main source of run production. His on-base percentage was generally only 30-40 points higher than his batting average. Getting on base wasn't a priority for Bell—his ability to plate runs was, however.
The role of the cleanup hitter isn't likely to change as baseball continues to evolve, so each team looks for that one main run producer who can best fill that role in the future.
Here is a look at who the cleanup hitter for each MLB team will/could be in 2015.