Viewing one of the most iconic paintings in San Francisco, by one of the Bay Area's most acclaimed artists, requires a nocturnal stroll down a stretch of Market Street crisp with crack vials, where streetside prostitution is the primary cultural attraction. You won't see this artwork during the daytime. Spray painted on the roll-down door fronting New Step Fashion, on Market near Sixth Street, the picture is cranked up out of sight during business hours, to reveal heaps of discount T-shirts and sneakers.

Barry McGee's work has been shown locally at Gallery Paule Anglim and SFMOMA, and nationally in museums from Manhattan to Minneapolis. But his simple, unforgetable black-and-white portrait of a downcast Everyman-in-undershirt has, since he created it one night in 1991, drawn countless thousands to this squalid corner. Some viewers are street artists in their own right, admirers of McGee's graffiti, which he used to sign "Twist." But many are art world cogniscenti, aware that the best McGee has to offer will never be found under halogen lights in a climate-controlled gallery. Conceived from street life, born on the run, his distressed figures belong to the urban landscape, interacting with the other markings on the walls, evoking accidental stories. The roughshod infrastructure is part of their habitat, and an essential element of his art.

This hasn't made McGee's figures especially welcome in their niche. Ever since New York mayor Rudolph Giuliani declared graffiti a "quality of life crime" in 1995, graffiti artwork has been grouped with spray can defacement and vilified as a social blights. In San Francisco, creating graffiti can be a felony offense, punishable with up to three years in state prison and $50,000 in fines. More appreciated than ever in galleries, graffiti artists are more beleuguered than ever on the streets, victims of Mayor Gavin Newsom's crusade to make the city Teflon clean. The threats have not noticeably reduced the amount of street art, nor has the estimated $22 million the city spends annually to erase anything that officials deem graffiti. As can readily be seen in two recent local films, Piece by Piece and Quality of Life, bombing the urban landscape with a spray can can be just as serious a creative endeavor as making poetry or opera or ballet, its practitioners willing to sacrifice comfort and safety for the sake of sharing their art.

Of course, the motivations underlying many scribblings are less lofty. Territorial gang markings and thoughtless vandalism are all too common. Nor is serious graffiti art consistently of high quality. Just as with oil painting or watercolor, graffiti art has myriad schools, styles, and techniques, from which emerges work ranging from the sparest monochrome tendrils to cathedrals of color. To argue over whether graffiti is inherently good or bad would be as meaningless as making such overarching claims about music or the Internet. Rather than fighting about enshrining or eradicating all graffiti everywhere, we'd be wise to consider a more complex and interesting question: What can graffiti art accomplish that no other artwork can?

A good place to begin looking for answers is Bluxome Alley, between Bluxome and Townsend Streets near Fifth. Pressed between two warehouses and barely wide enough to accommoate a Dumpster, Bluxome Alley is a prime stop on the international graffiti art circuit. Like the corner of Market and Sixth, it draws both practitioners and aficionados, but unlike McGee's roll-down-door portrait, the artwork here is constantly in flux. There's work right now by street artists from Oregon, Florida, and Canada. Most prominent is a 60-foot-long abstract mural by Vulcan, a spray can legend who recently moved to San Francisco from New York City.

Vulcan painted his first subway car in 1973, and by the '80s his underground reputation was such that his graffiti illustrated the cover of a Twisted Sister album. All these years on the street, often with police in pursuit, have taught Vulcan to be extraordinarily spontaneous. For this monumental piece on Bluxome Alley—a veritable hive of swirling color, incorporating 80 different spray paint tints—Vulcan's only preparation was sketching the two ends in pencil on a scrap of brown paper bag. The rest came as he worked in the alley over the next several weeks, mostly late at night.

This medium demands quick decisions. Linger with your spray can for an instant, and you may wreck hours of work. Stay with a piece for an extra minute, and you may spend a few years in prison. When you work on the street, as opposed to in a studio, each gesture is definitive, every decision irrevocable. Vulcan's major accomplishment, like that of a great jazz musician, is to communicate the feel of improvisation. Caught by surprise, we move our eyes swiftly from color to color, finding delight in pure invention. The spectacle is uninhibited by conceptual conceits, yet reveals the glittering thoughts of a lively mind. Vulcan's mural is simply, marvelously, a celebration of dead space redeemed by human inspiration.

That this piece won't last shouldn't disappoint us. The precarious state of art on the street demands that we view it with urgency, taking in what we can before the urban landscape changes again. Which isn't to say that Bluxome is a spray can free-for-all. Much like a museum, the alley is curated, albeit less formally, by an artist named Ricardo Richey. Known as Apex on the street, Richey has since the late '90s invited hundreds of peers to contribute their work to these two walls, layering the concrete with manifold lost masterpieces. He has also organized countless projects elsewhere in the city, including one last year with his group Gestalt Collective inside Yerba Buena Center for the Arts and another, updated annually, with artists Neon (who helps curate Bluxome Alley), Jase, and Chor Boogie on the side of City Discount Meat and Grocery, at Mission and 19th Streets.

The market's owners commissioned this piece as an antidote to the hit-and-run tagging that had previously market up the wall. Because the city obligates property owners to buff out vandelism and fines those who don't, tagging often causes substantial business expense. At City Discount Meat, Apex and his crew covered a thicket of hastily scrawled street signatures with multicolored serpentine abstractions that seem to writhe in three dimensions.

Aptly enough, the design incorporates the letters of the artists' own names. This technique, an elaboration on tagging, is known as wildstyle. It reaches back to the New York subways of the early '70s, when Vulcan was getting his start, and now can be found as far from the five boroughs as the Bolinas seawall, where young artists such as Richard Quevedo practice their lettering. Wildstyle tends to be so calligraphically complex as the be illegible to the uninitiated. Apex's lettering isn't elaborate for the sake of being elaborate, though. Rather, it is a game played with the viewer, as if his characters have come to life and are engaging us in their worldless mischief. The wall is no longer a barrier, solid and static—the colors burst through the concrete. To be on the street no longer has to mean being an outsider.

Permeating the municipal grid, skillful graffiti art makes the city less forbidding, and as the City Discount Meat project demonstrates, such work, given the chance to take hold, tends to overcome mere vandalism. If Vulcan's work suggests how the urban fabric can be essential to graffiti art, Richey's efforts as painter and curator illustrate how graffiti can be vital to the urban fabric.

Not that all spray can art is urban beautification. Some of it is willfully ugly, purposely threatening our comfort, confronting our complacency. The act of civil disobedience can be as important as the visual artifact: in conflict with authority, expressing far more than the blanket alienation and hate of vandalism, artful graffiti gains political traction of a kind that gallery art cannot.

The techniques of politically motivated graffiti artists vary, though the demands of mass communication make posters, stickers, and stencils particularly practical. All of these have been used by Southern California artist Shepard Fairey, who has arguably covered more acreage worldwide than any graffiti artist in history.

Fairey's street art carreer began in 1989, while he was a student at the Rhode Island School of Design. A friend asked him how to make a stencil, and to demonstrate, he began by tearing a promotional photo of professional wrestler Andre the Giant from a newspaper. The friend protested that the image wasn't cool enough, even for practice. Fairey cut the stencil anyway, slicing away background and shadows with an X-acto knife and tauntingly adding the line "Andre the Giant Has a Posse." Then he used the stencil to make stickers that he pasted up all over town.

Within days, Fairey overheard people speculating about the message. Was it from a skate-punk group? A political movement? A cult? The phenomenon intrigued him, and the more Andre the Giant ephemera he made, the more Fairey saw that it might become more than a prank. People started making their own stencils and posters of Andre. Over the next 15 years, image and message simplified—in part because of a lawsuit from the deceased wrestler's family—into a blunt, black-and-white icon of a brutal face with the words "Obey Giant" emblazoned beneath. That image has rendered the project more ominous with time, enlarging and entrenching the giant's sphere of influence. Whenever Market Street or the Mission is hit by Fairey or his many acolytes, both spectators and participants are impelled to assign meaning to a campaign with no purpose except sustaining itself.

Regularly appearing on prime advertising space, including billboards, the project is, of course, illegal. Yet despite several arrests on charges of vandalism, Fairey continues to infiltrate cities from San Francisco to New York to Berlin. As is often the case with political graffiti, the illegality is integral to the art. The questions Fairey asks about belief and persuasion are unsettling because, while mainstream political parties and corporations can easily purchase a city's visual space, that space remains inaccessible to the disenfranchised. While Fairey himself is not particularly disenfranchised these days—the popularity of "Obey Giant" has led to lucrative graphic design jobs for Toyota and PepsiCo—the guerrilla tactics ensure that Anre himself will bever become a sellout.

The truth is, Fairey's icon no longer needs him. The giant has taken on a life of his own. Like McGee's Everyman, he has become an autonomous character in the urban landscape, one of thousands to be found on city streets. The visual interaction of these characters, the stories suggested by their chance juxtapositions, may be graffiti art's most unexpected and meaningful quality.

On any given day, an observant San Francisco resident or visitor may encounter mutant bunny rabbits by Buff Monster, extraterrestrial pinup girls by Amandalynn, Frankensteinian castaways by Neckface, zero-gravity acrobats by Os Gemeos. The cumulative effect of the serendipitous, fluctuating relationships among these figures is epic, as if the city were a three-dimensional graphic novel, spanning storefronts and parking lots and bus lines, with no set page order or author. As the city changes, so goes the narrative.

It's a parallel universe, but the story is our own. We can let it flourish or, given enough beige paint and prison time, we can witness the making of a creative ghost town.

Behind these walls...
Getting a taste of graffiti art before you hit the streets.

FILMS  Quality of Life, a feature directed and cowritten by filmmaker Benjamin Morgan and shot entirely in the Mission, portrays two young friends who gain fame for their street art until the cops intervene. Piece by Piece, a documentary by University of San Francisco media and film studies grad Nic Hill, explores the local evolution of distinct graffiti art forms, from tagging to wildstyle, in the larger context of social and political debate.

EXHIBITION SPACE  Barry McGee shows paintings and installations at Gallery Paule Anglim (14 Geary St.); work by artists such as Neckface can be seen at 111 Minna; and street artists including Vulcan and Apex literally cover the walls at Hotel des Arts (447 Bush St.), invited by curator John Doffing to decorate guestrooms and public space.

WEBSITES Graffiti Archaeology (www.grafarc.org) lets you trace what's been painted in the train yards and back alleys of San Francisco over the years through the navigable pictorial time line. Startmobile.net is the latest venture from Hotel des Arts' John Doffing, who launched the site in December. An online gallery of dozens of street artists, from Bigfoot to Buff Monster, the site sells graffiti as wallpaper for cell phones—"new art for a new medium" at $1.99 a screen. To track the "Obey Giant" phenomenon and perhaps buy a T-shirt, there's www.obeygiant.com, "manufacturing quality dissent since 1989."

BOOKS  Still confused by the difference between street art and graffiti, tagging and postering? Street Art: The Spray Files, by Louis Bou (CollinsDesign), explores trademark styles and recurring characters in many of the genre's various forms through photographs and the occasional interview. —Martin Mulkeen