Participants’ understanding of consent information has been subject to rather intensive studies. This review was undertaken with the aim of exploring how understanding has been defined and measured in empirical studies of trial participants’ understanding of consent information. The databases Medline, Embase, ISI Web of Science, and PsycINFO were used to identify relevant literature published until October 2007. The following search terms were used: (informed consent OR consent forms) AND (clinical trials OR clinical research) AND (comprehension OR understanding). Thirty-five papers fulfilled the inclusion criteria and were reviewed in detail. Different terms were assigned to the variable: “understanding,” “comprehension,” “retention,” “awareness,” or “recognition.” A definition of what these terms implied was seldom given. Most of the studies used questionnaires for measuring the subjects’ understanding. The instruments differed substantially with regard to time of measurement, number of questions and which topics were covered. Most of the instruments were developed specifically for each study. The variation in definitions and measurements hinders comparisons of findings and thus improvement of consent forms based upon empirical studies.