A perennial problem for pop music reviewers is that by the time they're experienced enough to string an intelligible sentence together, they're often too old to relate to "the kids".

The world of emo (short for "emotional rock") is a case in point. Describing a loosely defined group of American punk-influenced acts, the word has been adopted by a whole generation of teenage music lovers for whom a melodramatic attitude and style of dress is just as important as musical taste.

While I can't claim to understand the passion and sense of unity London's emo kids displayed at last night's show - headlined and organised by emo kings Fall Out Boy and featuring their label mates - the musical attraction was obvious.

The much-vaunted Gym Class Heroes delighted the crowd with their intelligent take on hip hop, earning emo brownie points, it seemed, despite not playing anything remotely rock-like.

Led by Brendon Urie, Las Vegas rockers Panic! At The Disco showed from the start that they had far more to offer than their cool credentials.

In typical chilled-out surfer dude style, Urie wore his talent lightly, but couldn't hide a truly extraordinary voice - Freddie Mercurylike in its effortless ability to switch from delicate falsetto to gritty rockout mode.

I Write Sins Not Tragedies had the crowd chanting along to every word and raising their fingers in eerie unison, as did But It's Better If You Do.

The level of devotion provoked by Patrick Stump's Fall Out Boy, however, was nothing short of scary. As hyperventilating teenagers were whisked to safety by security at the rate of about one per five minutes, the group delivered a faultless, exciting display of accessible punk pop.

In keeping with his surname, the singer sported a physique that was hardly typical for a pop star in a world of skinny teenagers in skinny jeans.

Despite this, the fact he barely moved, and the fact that he was wrapped up in a heavy leather jacket and a cap that all but covered his face, Stump exuded calm charisma as he soaked up the love of his finger-waving fans.

I still don't know what emo is all about, but if they're listening to music as good as this, they must be doing something right.