Baking in parchment is an easy way to get lots of flavor with minimal fat (see the CHOW Test Kitchen's recipe for Fish Baked in Parchment, pictured). Actual baking parchment paper isn't essential, though—you can adapt the method to aluminum foil, which also gives you the option of cooking your packets on the grill, foodieX2 notes on Chowhound.
But some 'hounds say parchment is superior to foil. While foil packets are easier to make, a neatly folded parchment envelope retains steam better. It puffs up in the oven, and slitting it open at the table makes for a dramatic presentation, weezieduzzit says. (Here's a short video demonstration of how to fold a parchment packet.) You can also skip the hand-folding and use parchment cooking bags, a shortcut that's "awesome for quick weeknight meals," weezieduzzit says.
Another problem with using foil is that it can react to acidic ingredients like tomatoes, white wine, and lemon juice, all great seasonings for packet foods, jungirl says. Leave it to Martha Stewart to come up with a hybrid solution: A product called Martha Wrap is actually parchment-lined foil, which means you don't have to choose.
Discuss: En Papillote - why parchment paper?
Photo by Chris Rochelle / CHOW.com