'Big data' presents many interesting opportunities and challenges. This article focuses on the potential use of social media sentiment analysis as a legitimate tool for criminological research to better understand public perceptions of crime problems and public attitudes to responses to crime. While a degree of scepticism should always apply to the use of unsubstantiated sources on the internet, SMSA is likely to be a rich source of valuable information. Observational SMSA research presents low-level risks in terms of human research ethics principally because the information derived is unlikely to lead to the identification of research subjects. It is arguable, but less certain, that material posted publicly online does not attract a reasonable expectation of privacy for the author. However, the strength of this argument may depend on the particular circumstances in which the material to be analysed was posted.