To write a history of our scientific knowledge of dream problems is so difficult because, however valuable some parts of this knowledge may have been, no progress in definite directions has been discernible. There has been no construction of a foundation of assured results upon which future investigators could continue to build, but every new author takes up the same problems afresh and from the very beginning. Were I to follow the authors in chronological order, and give a review of the opinions each has held concerning the problems of the dream, I should be prevented from drawing a clear and complete picture of the present state of knowledge on the subject. I have therefore preferred to base the treatment upon themes rather than upon the authors, and I shall cite for each problem of the dream the material found in the literature for its solution. But as I have not succeeded in mastering the entire literature, which is widely disseminated and interwoven with that on other subjects, I must ask my readers to rest content provided no fundamental fact or important viewpoint be lost in my description. Until recently most authors have been led to treat the subjects of sleep and dream in the same connection, and with them they have also regularly treated analogous states of psychopathology, and other dreamlike states like hallucinations, visions, &c. In the more recent works, on the other hand, there has been a tendency to keep more closely to the theme, and to take as the subject one single question of the dream life. This change, I believe, is an expression of the conviction that enlightenment and agreement in such obscure matters can only be brought about by a series of detailed investigations. It is such a detailed investigation and one of a special psychological nature, that I would offer here. I have little occasion to study the problem of sleep, as it is essentially a psychological problem, although the change of functional determinations for the mental apparatus must be included in the character of sleep. The literature of sleep will therefore not be considered here. A scientific interest in the phenomena of dreams as such leads to the following in part interdependent inquiries: