The Scale to Assess Unawareness of Mental Disorder (SUMD) is one of the most widely used instruments to measure insight into mental disorders. The aim of this study was to review all studies using the SUMD in the last 20 years. We performed an electronic search of MEDLINE using PubMed to identify all relevant studies published from 1993 to 2012. The following data were extracted from each article: characteristics of the SUMD (version, rating scale, scoring, and item/dimension used), methodological aspects (country, language, subject inclusion criteria, and sample size), and statistical methods to analyse insight. Of the 133 articles screened, 100 studies were included in the review. Fifty-two studies were published over the past five years. The SUMD was rarely used in its entirety, and the use of selected items or subscales was heterogeneous across studies. The studies also varied in terms of response modalities and in the use of 3- or 5-point Likert scales. The calculation of insight scores was highly variable and included the following: treating items as categorical or continuous variables, separate analysis of individual items, items expressed in terms of the sum total or the mean scores, and a range of score values used to define insight. This paper provides a systematic review of studies using the SUMD and reveals important differences in the versions used, the methods of calculation, and the interpretation of scores across studies. The use of a modified SUMD may compromise the psychometric properties of the scale, lead to erroneous conclusions, and prevents comparison of results across studies. Our review underlines the need for the standardised use of the SUMD.