Among the many moments of surprise that 2006 will bring, here's one of the first: How is it possible that Pig Iron, Philadelphia's beloved avant-garde ensemble, so full of youthful energy and imagination, is celebrating its 10th anniversary?

Pig Iron, still under the tripartite directorship of founders Quinn Bauriedel, Dito van Reigersberg and Dan Rothenberg, will mark the milestone in a big way. Between January and June, the company, in residential partnership with Drexel University, presents a season of five performance pieces (plus four evenings of cabaret). Included are some of their biggest hits.

They are starting near the beginning—1998's Gentlemen Volunteers—and it's a very good place to start. This expansive story of romance during World War I was a smash hit at the Edinburgh Fringe Festival. Suddenly, the company, not long out of college, was vaulted to international status. Rothenberg remembers the sense of passage. "Though none of us [the original members] were originally from Philadelphia, we were drawn together at Swarthmore by a common bond, wanting to know what was really going on in theater, and to be a part of it. Our first goal was Edinburgh, and we had a beautiful thing happen—a 5-star review from The Scotsman. There were lines around the block to see us."

If Swarthmore was Pig Iron's locus, globalism was its focus. Bauriedel mentions some influences: Polish theater, especially by Witold Gombrowicz and Tadeusz Kantor; and the clownish, highly physical work of Jacques Lecoq (several of Pig Iron's seven-member performing ensemble have studied at Ecole Lecoq).

It's an unusual blend, and helps explain the ensemble's range. Some Pig Iron pieces are heavy on language, while others are nearly wordless. The group has taken on the weight of world history (Poet in New York, a dramatic fantasia about Spanish playwright Federico García Lorca) as well as pop culture (Mission to Mercury, a song-and-dance tribute to the music of Queen).

Yet the ensemble-created works share commonalities. There's little conventional narrative; instead the emphasis is on a kind of theatrical impressionism ("intuition over story," is how Bauriedel puts it). Visual elements and music are hugely important to the group. Bauriedel wonders "whether we should continue to call what we do theater." On the other hand, he's interested "in expanding what theater is, where it sits in people's consciousness."

The decade-long collaboration has produced some memorable moments. Rothenberg mentions working on Shut Eye, a movement piece about sleep and dreams, in which they collaborated with legendary director Joseph Chaikin. "We had admired his work so much, and it was a real turning point. The effects of his stroke were still very evident, and he had few words. We needed to trust ourselves to interpret him. It made us more daring." Specifically, Chaikin asked if company member James Sugg could write some music. "James was a musician, but not a composer. Well, he needed to write something on the spot. That's what he did, and now he's become a composer!" (Sugg won the 2005 Barrymore for Emerging Theatre Artist.)

"When we took Gentlemen Volunteers to Romania," Bauriedel remembers, "we were originally booked into a nightclub, but ended up in a castle! In fact, it was a monument to soldiers killed in 1918, which is the time when Volunteers is set. The castle was on a hillside, and it took effort for the audience to get there—not just physically, but politically, too. This was during an authoritarian regime, when there was a lot that wasn't said. I like to think what we were doing is saying what wasn't said."

Performance spaces play a critical role in Pig Iron pieces. The company has created works for everything from stages to labyrinthian basements. How will they handle this at Drexel? "We're using their Armory for Volunteers; the others will be at the Mandel Theater, and in one case we're reversing the seating—the audience will be on the stage, and the performers in the house," says Rothenberg. Nonetheless, it's an inspired, if slightly surprising, collaboration. "We worked with a team of Wharton students, who helped identify possible spaces for our season. They helped find Drexel, where the people were very welcoming, saying, 'we have space but we need imagination.' Our season will create opportunities for Drexel students."

After 10 years, company commitment and enthusiasm for Pig Iron keeps growing. "I don't think I had a clear sense of style before Pig Iron," says van Reigersberg. "[The company] shaped me too deeply for me to trace it, almost—but I know that, with Pig Iron, I have had the opportunity to grapple with questions and invent approaches and refine precisely with a steady group of collaborators that I highly respect. And I am so grateful for that."

Gentlemen Volunteers, through Jan. 22, $15-$25, The Armory at Drexel University, 33rd and Market sts., 215-627-1883, www.pigiron.org.