– Why was Mr Cummings so important to Mr Johnson?
When Mr Johnson became Prime Minister in 2019, he controversially brought him into Downing Street as his top adviser.
In No 10, he was seen as the architect of the strategy which broke the deadlock in Parliament over Brexit and resulted in a crushing general election victory for the Tories.
– Why was he such a controversial figure?
From the start, Mr Cummings made clear his impatience with the traditional ways of Whitehall, promising to bring down a “hard rain” on the Civil Service.
One aide suspected of leaking was marched out of Downing Street by armed police on his orders while he appealed for “weirdos and misfits” to shake up No 10.
However it was his infamous trip to Barnard Castle in County Durham, in an apparent breach of lockdown regulations, which sealed his notoriety with the wider public.
– Why did he fall out with Mr Johnson?
There were reported strains last autumn with Mr Cummings said to be frustrated with Mr Johnson’s delay in ordering a second lockdown in England.
Matters came to a head when the Prime Minister chose the former TV journalist Allegra Stratton to take planned new televised daily press briefings.
The appointment led to the resignation of No 10 communications director Lee Cain – a close ally of Mr Cummings from their Vote Leave days.
Mr Cummings himself then dramatically departed after losing out in what was seen as a bitter internal power struggle with Mr Johnson’s fiancee, Carrie Symonds.
Mr Cummings is due to give evidence to MPs investigating the Government’s response to the pandemic on May 26.
He has already blasted the performance of the Department of Health and Social Care – saying it had been reduced to a “smoking ruin” – and there is likely to be plenty more in that vein when he returns to Westminster.