Age: 33
Nominated for: The Theory of Everything
The character: Stephen Hawking, the theoretical physicist and cosmologist who wrote the best-selling A Brief History of Time.
Oscar record: This is his first Oscar nomination.
The critics said: "Redmayne towers: this is an astonishing, genuinely visceral performance which bears comparison with Daniel Day-Lewis in My Left Foot. His Hawking starts askew - the glasses, maybe the shoulders a touch - and over the course of two hours contorts and buckles into a figure at once instantly familiar and fresh. This is more than just skilful impersonation - it's inhabitation. To look on as his face and body distort is to feel, yourself, discomforted, even queasy." Catherine Shoard, The Guardian.
Age: 52
Nominated for: Foxcatcher
The character: Carell plays John du Pont, an oddball billionaire who sets up an Olympic wrestling team.
Oscar record: This is Carell's first nomination.
The critics said: "From the beginning, you can't take your eyes off Carell; as if by some secret alchemy, the actor makes you believe that his character is an entirely uncharismatic man while delivering a completely charismatic performance. The combination of his thin, reedy voice with frequent heavy silences and odd vocal pacing is thoroughly unnerving." Todd McCarthy, Hollywood Reporter.
Age: 40
Nominated for: American Sniper
The character: Cooper plays Chris Kyle, a Navy SEAL sharpshooter who recorded more confirmed kills during four tours of duty in Iraq than any marksman in US military history.
Oscar record: This is Cooper's third Oscar nomination in as many years, having been shortlisted for best actor for Silver Linings Playbook in 2013 and for the best supporting actor award for American Hustle last year. As one of American Sniper's producers, he will receive an Oscar if Clint Eastwood's film wins best picture.
The critics said: "American Sniper hinges on Cooper's restrained yet deeply expressive lead performance, allowing many of the drama's unspoken implications to be read plainly in the actor's increasingly war-ravaged face. Cooper, who packed on 40 pounds for the role, is superb here; full of spirit and down-home charm early on, he seems to slip thereafter into a sort of private agony that only those who have truly served their country can know." Justin Chang, Variety.
Age: 38
Nominated for: The Imitation Game
The character: Alan Turing, the computer pioneer who played a key role in cracking the Enigma code used by the Nazis during World War Two.
Oscar record: This is a first nomination for Cumberbatch.
The critics said: "With a large dose of Sherlock and a sprinkling of The Curious Incident of the Dog in the Night-Time, Benedict Cumberbatch is just brilliant in the role of Alan Turing, the Second World War codebreaker in The Imitation Game. His Turing is as enigmatic - and often as mechanical - as the German encryption machine he lives to break. He is a tortured soul who only finds salvation in the exquisite predictability of numbers."  Kate Muir, The Times.
Age: 63
Nominated for: Birdman
The character: Riggan Thomson, a former movie superhero actor who hopes to revive his washed-up career by putting on a Broadway play.
Oscar record: This is the former Batman actor's first nomination.
The critics said: "It is one of those performances that is so intensely truthful, so eerily in the moment, so effortless in making fantasy reality, and reality fantasy, that it is hard to imagine Keaton will ever be better." Betsey Sharkey, Los Angeles Times.
A look at the best actor nominees for the 87th Academy Awards.