My 9 year old daughter usually wears both shirts and pants in a size 7-8 so I ordered medium and they fit perfectly - snug, but not tight. I bought them to wear under my soccer jersey and they fit perfectly. She has sensory issues related to clothing and finding clothes to wear, especially pants, is almost impossible. She put them on without fuss and told me they were amazing. The fabric inside is soft, the seams do not interfere, the belt is comfortable and "does not press on the stomachโ.