When shopping for a sweatshirt, check out what materials are being used. Most sweatshirts are made using some form of polyester blend (e.g., 85% polyester blended with 15% cotton or 50/50 poly/cotton blend), though you can find 100-percent cotton sweatshirts. If you're looking to stay cooler during summers or heavy workouts, the more cotton in the fabric, the cooler it will be. Conversely, for cold-weather wear, look for sweatshirts with a higher percentage of polyester. Some makers even blend in wool for additional warmth. One thing about polyester is that it tends to hold onto odors much more than cotton does. If you plan on doing a lot of sweating, go for a higher blend of cotton.