There are a few different shirts that you can wear under the Hot Shot Onesie, largely dependent on the weather. In warmer months just wear a sleeveless tank or crop top. You can also wear a T-shirt with short sleeves. When it's a bit cooler, a long sleeved T-shirt looks good with the Hot Shot Onesie. You'll definitely need to wear something underneath it as it can be a bit too revealing otherwise.