New Orleans' carnival season - which starts on Twelfth Night and runs for the six weeks or so until Ash Wednesday - is unlike any other in the world. Though the name is used to define the entire season, Mardi Gras itself, French for "Fat Tuesday," is simply the culmination of a whirl of parades, parties, street revels and masked balls, all inextricably tied up with the city's labyrinthine social, racial and political structures. Mardi Gras was introduced to New Orleans in the 1740s, when French colonists brought over the European custom, established since medieval times, of marking the imminence of Lent with masking and feasting. Their slaves, meanwhile, continued to celebrate African and Caribbean festival traditions, based on musical rituals, masking and elaborate costumes, and the three eventually fused. From early days carnival was known for cavorting, outrageous costumes, drinking and general bacchanalia, and little has changed. However, although it is the busiest tourist season - when the city is invaded by millions - Mardi Gras has always been, above all, a party that New Orleanians throw for themselves. Visitors are wooed, welcomed, and showed the time of their lives, but without them carnival would reel on regardless, dressing wildly, drinking and dancing its bizarre way into Lent.

It was in the mid-nineteenth century that official carnival took its current form, with the appearance in 1857 of a stately moonlit procession calling itself the Krewe of Comus, Merrie Monarch of Mirth . Initiated by a group of Anglo-Americans, the concept of the " krewes ," or secret carnival clubs, was taken up enthusiastically by the New Orleans aristocracy, many of them white supremacists who, after the Civil War, used their satirical float designs and the shroud of secrecy to mock and undermine Reconstruction. Nowadays about sixty official krewes equip colorful floats, leading huge processions on different - often mythical - themes. Each is reigned over by a King and Queen (an older, politically powerful man and a debutante), who go on to preside over the krewes' closed, masked balls. There are women-only krewes; "super krewes," with members drawn from the city's new wealth (barred from making inroads into the gentlemen's club network of the old-guard krewes); gay krewes and important black groups. The best known of these is Zulu , established in 1909 when a black man mocked Rex, King of Carnival, by dancing behind his float with a tin can on his head; today the Zulu parade on Mardi Gras morning is one of the most popular of the season. There are also many alternative, or unofficial krewes , including the anarchic Krewe du Vieux (from Vieux Carré, another term for the French Quarter), whose irreverent parade and "ball" (a polite term for a wild party, open to all) is the first of the season. And then there's the parade of the Mystic Krewe of Barkus , made up of dogs, hundreds of whom, during what is surely the campest parade of the season, can be seen trotting proudly through the French Quarter all spiffed up on some spurious theme.

Tourists are less likely to witness the Mardi Gras Indians , African-American groups who, in their local neighborhoods, organize themselves into "tribes" and, dressed in fabulous beaded and feathered costumes, gather on Mardi Gras morning to compete in chanting and dancing. As in Sydney and Rio, the gay community also plays a major part in Mardi Gras, particularly in the French Quarter, where the streets teem with strutting drag divas.

One important New Orleans Mardi Gras ritual is the flinging of " throws " from the floats. Teasing masked krewe members scatter beads, beakers and doubloons (toy coins) into the crowds, who beg, plead and scream for them. Souvenirs vary in worth: the bright, cheap strings of beads are least valuable, while the bizarrely garbed coconuts handed out by Zulu are worth their weight in gold. When the parade is over, tourists embark upon a frantic bead-bartering frenzy, which has given rise to the famed "Show Your Tits!" phenomenon - young co-eds pulling up their shirts in exchange for strings of beads and roars of boozy approval from the goggling mobs. Anyone keen to see the show should head for Bourbon Street.

The two weeks leading up to Mardi Gras are filled with processions, parties and balls, but excitement reaches fever pitch on Lundi Gras , the day before Mardi Gras. Some of the city's best musicians play at Zulu 's free party in Woldenberg Park, which climaxes at 5pm with the arrival of the king and queen by boat. Following this, you can head to the Plaza d'España , where, in a formal ceremony unchanged for over a century, the mayor hands the city to Rex, King of Carnival. The party continues with more live music and fireworks, after which people head off to watch the big Orpheus parade, or start a frenzied evening of clubbing. Most clubs are still hopping well into Mardi Gras morning.

The fun starts early on Mardi Gras day, with walking clubs striding through uptown accompanied by raucous jazz on their ritualized bar crawls. Zulu, in theory, sets off at 8.30am (but can be as much as two hours late), followed by Rex. Ironically, by the time Rex turns up, many people have had their fill of the official parades. The surreal St Ann walking parade gathers outside the R-Bar at around 11am, while the gay costume competition known as the Bourbon Street awards gets going at noon. In the afternoon, hipsters head to the Faubourg, where Frenchmen Street is ablaze with bizarrely costumed carousers. The fun continues throughout the Quarter and the Faubourg until midnight , when a siren wail heralds the arrival of a cavalcade of mounted police that sweeps through Bourbon Street and declares through megaphones that Mardi Gras is officially over. Like all good Catholic cities, New Orleans takes carnival very seriously. Midnight marks the onset of Lent, and repentance can begin.

Other New Orleans festivals

St Joseph's Day , March 19. Sicilian saint's day, at the mid-point of Lent. Families build massive altars of food in their homes, inviting the public to come and admire them and to share food. The Sunday closest to St Joseph's ("Super Sunday") is the only time outside Mardi Gras that the Mardi Gras Indians take to the streets.

Spring Fiesta , March/April (week after Easter). Five-day festival when many of the loveliest homes in the French Quarter and Garden District are open to the public. It's all rather genteel, with guides rigged up in hooped skirts and a classical concert series. Contact tel 504/581-1367.

French Quarter Festival , early April. Free three-day music festival that rivals Jazz Fest for the quality and variety of music on offer. Stages and food stalls, a jazz brunch in Jackson Square, tours of private patios, free evening gigs, parades and talent contests. Contact tel 504/522-5730 or .

Jazz and Heritage Festival (Jazz Fest) , end of April/start of May at the Fairgrounds Race Track. Fabulous, enormous festival, with stages hosting jazz, R&B, gospel, African, Caribbean, Cajun, blues, reggae, funk, Mardi Gras Indian and brass band music, with evening performances in clubs all over town. Also crafts and fantastic food stalls. Contact tel 504/522-4786 or .