The mugs are made of thick glass but there is some heat transfer when dealing with hot liquids. For a normal cup of coffee the sides will get slightly warmer (perfect for cold days!). However, if you were to pour boiling water into the mug (for your Black and Herbal teas), the sides would be a little too hot to handle. But have no fear, the handle always stays cool!The mugs are microwave safe for heating drinks.