Ralph Lauren thin ribbed knit. Stretches well and is very easy to put on. As an inner layer in winter. Looks great worn alone over underwear in autumn and spring. 24M I think it will fit a 2-year-old without any problems. Even a 1-year-old can wear it with the sleeves folded up, and the body should be fine. It's incredibly versatile. Other colors are also available. Discounts for combined purchases.