It is tempting to use the Handicap-Santé survey, conducted in France in 2008 and 2009, to estimate the proportion of the general population affected by "mental disability," and to describe its characteristics and support needs. This article aims to show that improving our understanding of mental disability is a delicate task, even when the collected data cover the entire spectrum of the phenomenon, from illness to environmental factors. The primary source of difficulty stems from the lack of consensus on the very definition of mental disability. Furthermore, the very nature of the disability and the qualitative dimension of the impairments or activity limitations experienced sometimes make it difficult to answer questions with discrete response options. Analysis of mental health data regarding illnesses, impairments, and activity limitations shows that there is only partial overlap between these different categories of data, which may sometimes stem from a lack of reliability in the collected data. Nevertheless, the wealth of information collected allows for insightful descriptions of mental health issues, even if this does not exactly correspond to expectations regarding the concept of “psychological disability.” (R.A.)