People reveal a lot about themselves on social media, including information they would prefer to keep private. This information is discovered using advanced computer programs or algorithms, which examine the huge amounts of data people share online and look for subtle patterns within it. From that, they are able to predict or infer a wide range of personal attributes (including things like sexual orientation, personality traits, drug and alcohol use, and political leanings). In some instances, they can even predict a person's future behavior. This chapter describes some of the work happening in this area and insights that may come within reach in the future.