Size up if you are between size measurements like me. I got a medium and I'm 5'3" 120lbs. and it is snug with no gapping and good coverage. I like the way it is cut and the fact that the straps are adjustable.
This leotard is comfortable, fits well, and seems to be good quality. The front is lined but the back is the slightest bit sheer. I ordered it in light pink, thinking it was the color in the picture, and was disappointed that it is actually much lighter. It matches well with "theatrical pink" tights though, I think it would be very cute on younger girls.