As Brown's economic adviser, Balls famously penned an early Brown speech discussing 'neo-classical endogenous growth theory'. He was made MP for Normanton in 2005 and entered the cabinet last year.
What is his background?
Educated at Nottingham High School, Balls went on to attend Keble College Oxford. He was a journalist on the Financial Times before joining Brown's shadow treasury team. He is married to Treasury chief secretary Yvette Cooper.
Where does he sit politically?
As a staunch Brownite, Balls shares Brown's commitment to reducing child poverty. In recent months, Balls has made a strong play to the left of the party. He has attacked selection in schools and accused grammar schools of elitism.
Why he is a man to watch
At the DCSF, Balls oversees a wide-ranging brief and has a big budget to play with. As Brown's closest political ally, he is also a key influence on the direction of government policy in general. And he is hugely ambitious. When Brown goes, Balls could well be his anointed successor.
Things you might not know
It was recently revealed that Balls spent a term at Eton when his father taught there briefly. Balls was too young to attend the school, however, and insists he went to the local primary.