These two dishes are very similar, and in many American households, the words are used interchangeably.  Since there are so many different variations of both recipes, it's easy to get them confused. Scalloped use thinly sliced spuds, are cooked in a creamy sauce infused with garlic and herbs, and are traditionally made without cheese. Au gratin also uses very thinly sliced spuds, can be cooked in a creamy sauce, are always made with cheese, and can have a crunchy bread topping.