The main festival, celebrating its 50th birthday, runs alongside its naughtier sibling, the Fringe, which has grown almost out of control with 1,278 shows all over the city. The ever-expanding Book Festival is housed in marquees in Charlotte Square Gardens and the Film and Television Festivals are going on in the wings. And, for the militaristic, the Tattoo is booming away at the castle.

The Fringe is for the young - well, the energetic anyway. Take a walk down the Royal Mile or around the Mound, near the National Gallery of Scotland, and you begin to wonder if there is a single person not desperate to dress up, show off and draw a crowd.

In the streets children try to distract living “statues”, gaze at Punch and Judy, attempt to sort out the melancholy piper from the trad jazz and bongo drums around the corner or do some publicity for their own shows. Annie sings her heart out in the street (there are two competing productions of the saccharine musical) and a circus troupe passes by casually on stilts. In an overheard conversation it emerges that Hypno the Amazing Dog, part of a hypnotism act, is homeless - his landlady suddenly noticed that he wasn’t a puppet.

Edinburgh is the place to try out your theatre skills, to experiment. The Fringe audience complies and has a good-humoured taste of this and a bite of that - perhaps five events a day. Some shows never play to more than two lost tourists, some acquire the buzz of success, aided by the daily updating of Festival news and reviews. Shakespeare for Breakfast is a success - you have to book a day in advance and the sweltering room at the top of the Royal Overseas Club (C venue) in Princes Street is always packed - perhaps 80 or 90 people. A cast of six have fun, sweat flying off their faces, with “heritage” Bard. What would happen if Cordelia agreed with Goneril and got a share of Lear’s kingdom? Let Macbeth’s Weird Sisters give a high yuck-factor cookery demonstrati on - eye of newt isn’t the worst of it. What is really going on when an RSC star takes a humble bow? It is irreverent and funny, rather like an end-of-term school satire in its assumption of shared knowledge. There’s the bonus of free croissants and coffee all round.

After a 25 minute break the same company, all in their twenties, are on stage again, this time in their children’s show, The Little Mermaid. The ensemble playing is just as strong, the tone now serious as Hans Andersen’s tale takes life. Opposite me a five-year-old, round-eyed, sticks his fingers in his ears while the Sea Witch warns the Little Mermaid of the dangers of meddling with human love.

Downstairs at C venue, half a dozen new graduates perform Shelf Life, a musical play about a paranoid shopkeeper, in a space the size of a prison cell. Here there is good singing and promising composition, but bravest of all, grand gestures under the chins of the heat-struck audience. And in the press office next door a teacher from Tottenham is spending his holiday slaving over a hot computer. Such is the pull of the Fringe.

There are dozens of shows for children, but it is the teenagers who have the time of their lives. There may be fewer British school and youth drama groups these days, but the international element is strong. The Hong Kong Youth Arts Festival have brought Curiouser... and Curiouser to the Famous Grouse House in Chambers Street. The Alice reference tempts some who are too young to appreciate this imaginatively choreographed dance sequence with words (some Chinese) about the agonies of adolescence and the mysteries of gender.

At St Oswald’s, a converted church at Montpelier Park, the American High School Festival has been part of Youth International. Their A (Rock-n-Roll) Midsummer Night’s Dream has had teenagers queueing impatiently in the sunshine and then clapping along to “Dream Dream Dream”,“Rock Around the Clock” and many other finger-clicking standards. Most of the words were Shakespeare’s; the humour and energy were all-American, from the world-weary, pink silk-jacketed fairies to bespectacled bean-pole Helena, the clutz Mechanicals and smoothy crooner Oberon.

Whole families have been intrigued by Sounds Sensational, part of the International Science Festival, where a willing dad had his voice raised several octaves by breathing in helium and we all experienced “surround sound”. This week science festival goers can catch Secrets of the Theatre - Explained and next week, Light and Illusion, all at the Famous Grouse House.

The National Student Theatre Company goes straight to the heart of young preoccupations with Sex and Death, a fast-paced black comedy with some outstanding acting. Psychological explanations for weird behaviour almost ruin the cleverly stylised presentation, however. Football mania leads to apparent accidental killings and farcical cover-ups - shades of Shallow Grave and Arsenic and Old Lace.

Theatre studies students - and anyone with a sense of humour - should rush along to the Edinburgh International Conference Centre where a company from Barcelona, La Cubana, have colonised the building with Blinded by Love. This is a truly multi-media event. You make your way through an exhibition of kitsch tableaux designed to explain, tongue-in-cheek, Valencia, Catalunya, infant acting prodigies and Spanish Holy Week celebrations (a doll Virgin flanked by serried ranks of candles) to a northern Protestant audience.

A rasp-voiced attendant warns loudly that this is the last chance to visit the loo as each audience member is handed a pair of heart-shaped 3D specs. Soon we are watching a film - a spoof 60s romance in which the heroine is tragically blinded by pigeon shit. But the real story turns out to be the infighting between the people making the film. Or rather the real story turns out to be happening in the theatre and to involve us. With astonishing slickness and faultless timing, characters step out of the screen and back again, interacting with those left behind and with the audience. There is a serious debate going on somewhere underneath all this about the nature of reality, about the truthfulness of representation on stage and screen, but the pace never falters and, by the end, the audience is crying for more - even those who have been inveigled into dressing as Penitents in flowing purple robes.

La Cubana are part of the Festival proper, but the Fringe is no less international. Shakti is a Japanese-Indian dancer performing The Tibetan Book of the Dead at Caf Graffiti, a majestic but crumbling abandoned church. Outside, in the churchyard, you can eat and drink into the night; inside, you are invited to contemplate a Buddhist afterlife as Shakti, wearing zipped rubber or gauzy silk (costume changes happen on stage), takes us on a tightly-choreographed journey from Death to Life. She is powerful, even aggressive, ably backed by the whirling Vasantamala Dance Company from Tokyo. The music is otherworldly. Only a falsetto “Amazing Grace” comes as a bit of a surprise as the golden eggs of rebirth are rolled towards us.

However many shows you pack in you will be convinced you have missed a plum somewhere else - and you probably have. But, even if some of what you see is off key, the sense of being in a cauldron of creativity never fails to be exhilarating. I’m sure I could do a one-woman show about Enid Blyton or Queen Victoria or what about Ibsen at tea-time . . . ?

Festival box office: 0131 473 2000

Fringe box office: 0131 226 5138