My grandchildren needed these gold shirts to go under their club soccer shirts . These were just right and they loved them. I did order up a size since they fit snugly and they will be able to use them next year also.It's true, the fit is very, very slim and fitted. However, I bought it to keep my son and daughter warm under their baseball uniforms. The snug fit keeps their jerseys from bunching up while tucked into their pants. However, my kids are very lean, so if your child has a larger frame I can see why some feel they are way too fitted. Also, my children have long arms and the arms are quite long. We have to fold up the sleeves a bit. The shirts do the job well in our case.True compression. I'd been looking for a shirt like this for my son with sensory processing issues. The ones I found in the stores were either long enough (to tuck in) and too loose or tight enough but only came down to his belly button. These are deliciously long and delightfully compressive. My son is 6 years old, 50" tall and 51lbs. That's tall and downright skinny. He actually likes the fit of the small even though it fits like a second skin. For most, a size Medium would be perfect. Ordering more today.Ordered this for my almost 4 year old to wear under his FLASH costume. As expected, sleeves a smidge long - but nice snug fit for my 40lb 3.5 ft tall boy. Would be perfect for 4-6 year olds. Just need to stretch the neck hole to get it over their heads - once it's on - fits like a second skin as it should. It has a high crew neck - but not a mock turtleneck.I read all the reviews that they ran small so I ordered a size up and it was too big. These seem to run exactly as sized. Otherwise, great shirt. She'll just have to grow into it.After reading other reviews, I purchased 2 of these for my 8yo nephew. He normally wears a medium but I ordered a large as reviewers stated these run small. After the first wear on the way home from a game my nephew said, "I like this shirt! It's comfortable!" I think there's no better testimony than that!I bought a large thinking it would be more of a 10-12. However it's more like a 8-10. It fits this season , but I should have bought a bigger size. Works well, just as good as an underarmour shirt.Based on previous reviews, I chose to get this shirt a little larger since I did not want to return it. It was hard to get over my son's head but once it was on, it fit perfectly.