Poly/viscose/lycra blend jersey. This dress is amazing. To make a dress like this with so little investment of time or materials: thank you, Vogue.
I made a size 10. I usually make a 6 or 8, but I didn't want it to fit too closely.
I used a piece of black jersey from my stash, but didn't have quite enough, so I bought an additional 5/8 yard of horizontal striped fabric for the back piece. I like the way it turned out---you can see the nifty curved back seam with the contrasting fabric.
I might sew this again. It's a little clingy for me, so I'll have to get up the nerve to wear it! We'll see.