To date, in Western jurisdictions, many criminal justice reforms are devised and implemented with a close eye on public opinion. These are typically intended to regain or foster legitimacy. However, within this context, there is no common understanding of this concept. This essay aims to provide such a conceptualisation of legitimacy, to enable a consistent and systematic evaluation of attempts to accommodate public opinion. To this end, five levels of legitimacy research are discussed that could structure evaluations of public-opinion-targeted reforms: (1) the normative dimension, (2) the ‘audiences’ addressed, (3) the purpose of the reforms, (4) trust and distrust, and (5) dialogic, or longitudinal effects. Furthermore, since research departing from these five levels of analysis is likely to result in observations that are, by nature, incommensurable, it is argued that an overall assessment of legitimacy always requires a judgment, rather than mere measurement.