Castro's struggles with Washington have seen him become an icon of international socialism during his 47-year-rule.
During that time the US has imposed a crippling economic blockade of Cuba, backed an attempted invasion by Cuban exiles and made numerous attempts to assassinate him - including by giving him a cigar packed with explosives.
Castro, a charismatic figure known for his long and fiery speeches, has enjoyed a revival in international support in recent years.
At the same time the rise to power of governments sympathetic to socialism in Venezuela, Bolivia and across Latin America has seen Cuba return to favour in the region.
"I'm really happy to reach 80. I never expected it, not least having a neighbour - the greatest power in the world - trying to kill me every day," he said during a recent trip to Argentina.
Castro's Cuba has been held up as inspiration to developing countries that they could take on the might of the US and get away with it.
He has also won friends by sending 20,000 Cuban doctors to help the poor in countries as far afield as Pakistan and East Timor.
Castro has close ties to
Venezuela's Hugo Chavez.
Around 260,000 eye patients have recieved free treatment in Cuba in a joint program with Venezuela that began in 2005.
Castro's opponents, however, have vilified him as a totalitarian ruler prepared to use draconian methods to suppress dissent inside Cuba.
In 2005, Amnesty International accused the Cuban government of holding over 70 dissidents in poor conditions only because they disagreed with the government.
Castro was born in August 1926, the son of wealthy Spanish immigrant landowner.
In his youth, Castro was known as a promising baseball player who dreamed of playing in the US big leagues.
He began his revolutionary career with an attack on a military barracks in the eastern Cuban city of Santiago in 1953.
Castro (right) and Che Guevara
led a guerrilla army
Castro was arrested but later freed under an amnesty and travelled to Mexico where he organised a rebel movement that launched another ill-fated attempt at revolution in Cuba in 1956.
Most of his followers were killed or captured, but Fidel and his popular comrade Che Guevara escaped and eventually drove Fulgencio Batista, the pro-American Cuban president, into exile in 1959.
Only sketchy details are known about Castro's private life. He has married three times and has seven children.