Have you ever been swayed either positively or negatively after an ad? Has an ad ever changed your mind about a brand? Online services have upended the world of advertising by enabling marketers, including political campaigns, to refine their pitches to specific groups of people or geographic areas. And in the age of Big Data, it has also created new problems, “policy vacuums”, and unforeseen challenges. Welcome to the age of microtargeting. We explain how this works.