This paper presents an equilibrium theory of product complexity. Complex products generate higher potential value, but require more attention from the consumer. Because consumer attention is a limited common resource, an attention externality arises: Sellers distort the complexity of their own products to grab attention from other products. This externality can lead to too much or too little complexity depending on product features and the consumer's attention constraint. Products that are well understood in equilibrium are too complex, while products that are not well understood are too simple. Our theory sheds light on the absolute and relative complexity of different goods, including retail financial products.