The purpose of this article is to review briefly a few of the many questions that have arisen directly or indirectly out of the interest taken of late years in the matter of the care and control of the feeble-minded, and to comment on some of the aspects of the legislative measure, known as the “Mental Deficiency Act,” which has ultimately resulted from that interest. Some reflections by the way on this subject, about which at the present moment there is necessarily but little practical activity, may, I trust, be regarded as not wholly inopportune. In offering to the readers of this Journal the following remarks on some difficulties that have been raised regarding the interpretation and working of the new Act, and in noting certain misconceptions and more or less irrelevant discussions that seem to have obscured the practical side of the subject, I have but the excuse of a long-continued interest in the matter of the due recognition of mental failure in all its varieties, and of my personal experience, gathered from the study of criminals, touching the part played by mental defect or disorder as a factor in the production of crime. These notes, which are at least meant to be practical, will therefore tend to circle chiefly round the questions of the actual diagnosis of such mental defect in persons of all ages as is now officially registered under the term “mental deficiency,” and of the relation that seems to exist between mental defect generally and criminality.