However much you think their days should be over, the Pet Shop Boys just won't lie down.

This week a new display opened at the National Portrait Gallery charting the duo's seemingly immortal pop career through the iconic images which define their frequent reinvention.

Marking the twentieth anniversary of their first album, it coincides with the release of a new live album, Concrete, as well as the publication of Catalogue, a book about the Pet Shop Boys' style journey.

All this after releasing their ninth studio album, Fundamental, in May.

The pathway to success for the Pet Shop Boys began in 1985 when Neil Tennant quit his job as deputy editor of pop magazine Smash Hits to join up with architecture student Chris Lowe.

Other popsters of the same generation have long since died a death - Rick Astley, The Housemartins, Bananarama, to list but a few.

But the Pets went on to become the most successful pairing in British pop history, with 36 top 20 hits and ten top ten albums to their name.

Yet now, despite balding and middle-aged, the band seems far from mid-life crisis, lapping up collaborations with Robbie Williams and Rufus Wainwright and now producing a Christmas single for Matt Lucas.

Pretty impressive for a band which, by their own admission, "came on stage and didn't do anything".

So how has this act, known for its ironic combination of disco beats and motionless stage presence, sustained its popularity?

The answer can in part be found in this gallery of portraits which captures on the one hand the Pet Shop Boys' continually changing style and, on the other, the tongue-in-cheek, morose persona which has been the constant thread through the past two decades of their existence.

From the beginning, the band's image has been, in Tennant's own words, "not quite but almost of equal importance" as the music itself.

Among the artists, film-makers, designers and photographers they have collaborated with are Derek Jarman, Sam Taylor-Wood, Mark Farrow, Bruce Weber and Wolfgang Tilmans.

This particular exhibition is mostly made up of the work of Eric Watson, known by many as "the third Pet Shop Boy" for the integral part he played in their development.

In the early days, on the covers of West End Girls and Rent, Watson depicted the duo as "real" characters, photographed wearing deliberately ordinary clothing and shot in gloomy lighting on the platform of Kings Cross Station or in a dingy Wandsworth street.

Most commonly Tennant stands in the foreground dressed in a dark suit while the more laddish Lowe is behind in cap and glasses.

In the 1990s they transmogriphied into computer-game-inspired puppets dressed in orange jump suits and wearing striped conical hats, but still with the same dead-pan expressions on their faces.

Throughout the journey, the boys seem to have revelled in the contradiction they present - such a serious-looking pair making boppy, upbeat disco - never afraid to poke fun at the pop industry or at themselves.

Like these portraits, their lyrics often contain both gravity and mockery. And like their image, their music has evolved over time but retained its inimitable sound.

This small collection is just a snap-shot of the photos and trivia contained in the Pet Shop Boys Catalogue and, tucked away in the Bookshop Gallery, it could easily be missed if you're mooching round the main exhibition areas.

But for anyone interested in pop history, this free display is well worth a visit and is a welcome addition to the gallery's collection of iconic 20th century portraits.

  • Pet Shop Boys Portraits runs from October 30, 2006 to February 28, 2007 at the National Portrait Gallery.