Delinquency is one of those vague but fashionable words that are becoming very difficult to define. Originally it appears to have been used to describe minor infringements of the social, moral or legal code, with special reference to acts of omission rather than acts of commission. It is now often used to describe any kind of anti-social conduct, whether trivial or serious, while some use it as a harmless euphemism for crime, although the word crime, in its technical meaning of an act or omission punishable by law, is inoffensive, and has no reference to the question of moral guilt or culpability. It is therefore perhaps advisable for me to say what meaning I attach to the word to-day. For the purpose of this paper I am using the word in the general sense of criminality, that is to say, I am confining my remarks to the more serious types of anti-social conduct shown by mentally defective persons.