“Half the money I spend on advertising is wasted; the trouble is I don’t know which half.” When the legendary 19th-century merchant John Wanamaker said this, he captured the angst of generations of business owners. Marketing and advertising clearly work—but how, and to what extent?

 Performance marketing aims to solve this tension by focusing on the part of marketing that performs in a measurable way. The term, first introduced in the mid-1990s shortly after internet marketing, was a stroke of branding genius by marketing companies. If they have the choice, why would a business owner invest in anything other than marketing that performs?