This Essay describes emerging big data technologies that facilitate horizontal cybersurveillance. Horizontal cybersurveillance makes possible what has been termed as “sentiment analysis.” Sentiment analysis can be described as opinion mining and social movement forecasting. Through sentiment analysis, mass cybersurveillance technologies can be deployed to detect potential terrorism and state conflict, predict protest and civil unrest, and gauge the mood of populations and subpopulations. Horizontal cybersurveillance through sentiment analysis has the likely result of chilling expressive and associational freedoms, while at the same time risking mass data seizures and searches. These programs, therefore, must be assessed as adversely impacting a combination of constitutional rights, such as simultaneously affecting both First and Fourth Amendment freedoms.