The Big Question: Why does Bob Dylan keep touring and is he still the best?

The living legend is back in the UK

Why are we asking this now?

Because, like God, Dylan is everywhere. His ubiquity is extraordinary. His 33rd studio album Together Through Life will shortly be released, the fourth in an extraordinary late flowering of bluesy songs that kicked off with the brilliant Time Out of Mind in 1997. His Theme Time Radio Hour, available here on BBC6, has logged 100 hours of quirkily eclectic music from a slew of genres, even if it may be coming to a close (his most recent song "theme" was "Goodbye"). His recent exhibition of paintings, the Drawn Blank Series, in London's Mayfair may be followed by a travelling sculpture exhibition in Europe next year. This Sunday's one-off concert at the Roundhouse is a stroll in the park for a man who routinely performs 150 concerts a year. And if anyone ever mentions the world's most prestigious writing award, the Nobel Prize for Literature, somebody will tap his nose and sagely assure you that Bob Dylan has been "on the shortlist" for the last four years. Oh and Barack Obama brags about having Dylan's songs on his iPod. Like I say, ubiquitous.

Remind me: who is Bob Dylan?

Born Robert Zimmerman in May 1941. Family descended from Russian and Lithuanian Jews. Raised in Duluth and Hibbing, Minnesota, where formed bands in high school. Dropped out of University of Minnesota, determined to infuse US folk music with new seriousness. Went to New York, discovered art and books, sat at hospital bedside of his hero, Woody Guthrie. Began to perform songs in Greenwich Village. First album of cover versions from Columbia, 1962. Made reputation with second and third albums, The Freewheelin' Bob Dylan and The Times They Are A-Changin', as musical seer and prophet of social breakdown and political apocalypse in songs such as "Blowin' in the Wind", "Hard Rain's Gonna Fall", "Masters of War", "Chimes of Freedom", etc. His sandpaper rasp and unmelodious whine put off some listeners, but cover versions by Joan Baez, The Byrds and others showcased the melodies. His love songs and sardonic "talking blues" also impressed.

So he was the voice of the Sixties?

By 1964 he was considered the leading light of protest movement - but he soon rejected political rhetoric in favour of impressionistic, beautiful, image-driven songs of existential and cultural confusion: "Mr Tambourine Man", "Desolation Row", "Visions of Joanna", "Like a Rolling Stone". They introduced the concept of the long, thoughtful, poetic rock lyric and influenced everyone from The Beatles to Bruce Springsteen. But he irritated many folk fans by embracing electric blues and rock'n'roll in 1965.

Enough of the Sixties. That was ages ago. What happened in the Sixties, Seventies, Eighties and Nineties?

Dylan's finest period was 1963-66, three years of crazy fertility. In the Seventies, he hit a second stride with Blood on the Tracks and Desire, the first charting the end of his marriage, the latter returning to his early embrace of public political controversy with "Hurricane," about a wrongly -accused black boxer. The listening world sat up and took notice again. Dylan appealed to the stoned gypsy rover in his fans' hearts by embarking on the Rolling Thunder Revue with a dozen Greenwich Village folkies, commemorated in the documentary Reynaldo and Clara. He became a born-again Christian in the late 1970s and his output (Slow Train Coming, Saved) suffered.

The 1980s were a glum time: many albums flopped, his collaboration (eg with the Grateful Dead), charity singing (eg Live Aid 1985) and movies (eg the disastrous Hearts of Fire) were badly received. But he played new tricks. He revealed that he'd kept certain key recordings of the decade (like the epic "Blind Willie McTell") off his albums; they were later released as The Bootleg Series. He formed a casual, intensely melodic, folkie super-group called The Travelling Wilburys with George Harrison, Tom Petty, Roy Orbison and Jeff Lynne, to great acclaim. He also began the gruelling night-after-night gig schedule that became known as The NeverEnding Tour, which is still going strong.

In the 1990s, after another critical mauling, Dylan stopped making studios for seven years. He returned to his folk roots and made an album of classic blues and folk numbers. He needed a break. In spring 1997, he nearly died of a heart infection called pericarditis, but he bounced back to produce Time out of Mind later that year, his best-received work in years, which ushered in the current remarkable renaissance.

So the secret of his longevity is...?

Several things. 1) His transformation of rock'n' roll in the mid-1960s casts a long shadow: anything new that he does is greeted with respect. 2) His shifting of genres (folk, rock, country, jazz, western swing, rockabilly, lounge ballads, even rap – he invented the rap song with "Subterranean Homesick Blues") means he remains musically unpin-down-able. 3) The air of mystery and aggressively-defended privacy he projects, about his early days in Minnesota to his motorbike crash to his marital status, are red rags to critical bulls. 4) The lexicon of literary, Biblical and filmic hints with which his songs are studded have delighted successive generations of fans and academics, eg former Oxford Professor of Poetry, Christopher Ricks. 5) Lately, he has delighted hard-core fans by suddenly embracing the truth. His first volume of autobiography, Chronicles Vol 1 was a miracle of clarity and warmth about his early musical and literary education; Martin Scorcese's documentary about his life, No Direction Home, saw him giving straight answers to straight questions on-camera – something unheard of 20 years ago.

Isn't he just a prolix singer-songwriter who takes himself too seriously?

Actually, no. He is a modern version of the baffling, shape-changing riddler or trickster archetype from world mythology. He has played games with listeners, fans, cultists, academics, biographers and thousands of journalists over the years. As for his seriousness – whimsy has come to play a big part in his appeal. Kenneth Tynan used to say he was sure God the Father would be just like Ralph Richardson – a puckish, unpredictable, whimsical grandee. Dylan's the same. Listen to him name-checking young women singers on his last album ("I was thinking 'bout Alicia Keyes... I was wonderin' where Alicia Keyes could be,") watch his hat-and-cane soft-shoe shuffle in the video to his Oscar-winning "Things Have Changed," marvel at the way he lent his endorsement to the Victoria's Secret lingerie company, or recorded a Pepsi commercial with the rapper Will I.am broadcast at this year's Superbowl, and allowed the Co-Op to edit the lyrics to "Blowin' in the Wind" for a TV commercial, and you're aware of a man laughing at his own past and his reputation. He's one of the few undisputed musical geniuses of the 1960s explosion, but has always seemed able to laugh at his status as "voice of a generation."

Is Dylan worth all the fuss?

Yes...

* He's a living legend, who connects us with the very beginnings of rock and youth protest

* He has a bigger back catalogue of fine songs (600) than the Beatles, Stones and Beach Boys combined

* He can still be the conscience of America; he knows Barak Obama wouldn't want to let him down

No...

* Not any more. Listen to the backing of his new songs and you can tell he's lost interest in melody

* Have you heard him in concert mangling his old classics? It's a desecration

* He's only revered because he'll be the first 1960s rock star to hit 70...