What counts as knowledge in the age of big data and smart machines? Technologies of datafication renew the long modern promise of turning bodies into facts. They seek to take human intentions, emotions, and behavior and to turn these messy realities into discrete and stable truths. But in pursuing better knowledge, technology is reshaping in its image what counts as knowledge. The push for algorithmic certainty sets loose an expansive array of incomplete archives, speculative judgments, and simulated futures. Too often, data generates speculation as much as it does information. Technologies of Speculation traces this twisted symbiosis of knowledge and uncertainty in emerging state and self-surveillance technologies. It tells the story of vast dragnet systems constructed to predict the next terrorist and of how familiar forms of prejudice seep into the data by the back door. In software placeholders, such as “Mohammed Badguy,” the fantasy of pure data collides with the old specter of national purity. It shows how smart machines for ubiquitous, automated self-tracking, manufacturing knowledge, paradoxically lie beyond the human senses. This data is increasingly being taken up by employers, insurers, and courts of law, creating imperfect proxies through which my truth can be overruled. This book argues that as datafication transforms what counts as knowledge, it is dismantling the long-standing link between knowledge and human reason, rational publics, and free individuals. If data promises objective knowledge, then we must ask in return, Knowledge by and for whom; enabling what forms of life for the human subject?