In a competitive market, firms and consumers make decisions based on several aspects of the product involved. While price is a fundamental aspect, it is not the only one: in many markets, innovation, quality, variety or even privacy may play at least some role in the competition between firms. This paper introduces these characteristics, and the merger theories of harm associated with them. It also describes some of the practical challenges involved in considering non-price dimensions of competition, namely identifying when they are important, and considering price and non-price effects together. It was prepared as a background note for a discussion held at the OECD in June 2018 on non-price effects in merger control.