Teenage Riot and The Author @ Traverse

With a reputation for solid quality and experimental theatre that still appeals to a broad audience, The Traverse is a Fringe institution, and one of the rare Edinburgh venues that uses August as a logical extension of its general programme. Dominic Hill, willing to use the Traverse brand to challenge his audience, has undoubtedly booked two of this year's most exciting and controversial plays.

Ontroerend Goed, Belgian masters of the radical approach, veer away from last year's intimate Internal for a full-bore punk assualt on the audience. A character driven hour of teenage rage, it roundly attacks adults, celebrates adolescent rebellion in the bluntest terms and encourages a visceral response. After a series of monologues that insult and cajole, the young cast finally turn directly to the audience, film them, and pelt the images with rotten fruit. It is relentlessly aggressive, taking on anorexic stereotypes, teenage male sexual fantasies and imagining pubescence as a time of terror and insecurity.

Inevitably, the content is over-familiar: from the futurists of the 1920s, through rock'n'roll in the 1950s, via punk and then the slackers referenced in the Sonic Youth quoting title, the ghost of teenage rebellion against a stultifying adult world has been summoned. Despite the fierce rhetoric and confrontational script, Teenage Riot uncovers few surprises. What allows it to stand out is the sheer anger, the depth of the nihilism, and its direct communication to the audience. Walk-outs are common enough at the Fringe, but rarely so rapidly. People are knocking on the exits before an actor has graced the stage.

Then there is the sense of event: the audience seems to join in the action, chatting and roaring. The threat is palpable. During the performance, it is terrifying.

This terror may mellow after the show. Even the title suggests that the teenage angst does not really belong to the generation on stage: the script has the feel of being lifted from a generic set of complaints, with Daily Mail level discussions of violent video games thrown in for a contemporary edge. This is the voice of Generation X, spoken by the so-called Sunshine Generation. It's raw, tough and nasty, but leaves behind the last question of the Sex Pistols: "ever feel like you've been cheated?"

Tim Crouch shocks on a more subtle level. The Author is a play without a stage: Crouch and his cast sit in the audience, speaking directly about a play (that never existed) and experiences (that never happened). Crouch's distinctive voice - his characters speak with the gentle exaggeration of Fisher Price play-people - permits his story to plunge into depths of depravity and human horror, without the explicit shock tactics of Teenage Riot.

Simply by talking about the process of the play (that never existed), Crouch visits war-zones, internet pornography, live beheadings, social disorder and the supposed danger of theatre. Touching on the way that drama can rape real tragedy for a thrill, it winds towards a nasty conclusion that would fit a Greek tragedy.

The simplicity of the narrative and the primary coloured characterisation sanction Crouch's vicious dissection of our safety within the theatre: gut-wrenching moments are delivered in mundane, almost monotone, monologues. Yet it evokes the same terror as Teenage Riot, this time effortlessly.

Crouch has always been interested in the tragic, and he marries an almost classical simplicity of plot with a startlingly original take on the theatrical space. After Sarah Kane's sensory assaults of the 1990s, Crouch has moved forward the nature of violence within theatre: this is a deceptively profound, moving, difficult and important play.