I didn't know someone actually made underwear that could fit an athletic build and a really active life. These are so comfortable, they might even replace my soft cotton PJ'S for sleep. The legs don't ride up. There's no bunching fabric in my groin and the cotton stays dry all day. No rough seams in the middle of the most delicate parts. This has been my biggest pet peeve in women's underwear, even well-known, otherwise well-made brands. With that seam chaffing and rubbing all day, it feels like a form of torture. No more of that for me. For reference, I'm 5'7", 135 lbs, a competitive mountain biker. Waist is 29", so I got a medium. If I went by my hip size, it would be XS and then the waist would be a bit too small. Same if your hips are bigger; go by your waist size and you'll get a better fit.