We put these on the baby to wear under her sleep sacks. It's cold here, and she hates being swaddled, so I rest easy knowing she's warm and comfortable. Just note that these run small; she outgrew her 0-3 months within a few weeks of birth, and now at four months old she's just a little bit too big for the 3-6 months to fit comfortably. Anyway, I definitely recommend keeping these on hand, especially in the wintertime.