The traditional costumes worn during Cajun Mardi Gras are one of the most fun aspects of the day. Most costumes are homemade in a variety of bright colors. They're usually colored with patchwork and fringe, and they're often topped off with masks (hiding the identity of the rider, so they can really let loose) and hats, the shapes of which have been around for hundreds of years. The three main shapes for Cajun Mardi Gras hats date back to a time when villagers in Celtic France would use the anonymity of the Mardi Gras celebration to mock the elite members of their community - namely, the nobility, the clergy, and the intelligentsia.
To mock the nobility, paraders would wear tall pointed hats (known as capuchons), which resembled the hats that noblewomen would wear. To mock the clergy, they'd wear a mitre. To mock the educated class, a mortarboard. These days, the capuchon is the most popular hat shape for a Mardi Gras costume (possibly because it's the easiest to make at home), but the other shapes are still seen occasionally.
In this picture, you can see several costumed Mardi Gras participants, both in traditional costumes and some costumes with modern twists, being serenaded by musician Tony Davoren, one of the original musical cast members of Broadway's Riverdance.