The American Psychiatric Association and the American Association for the Advancement of Science jointly sponsored a symposium on hallucinations which was held at the annual meeting of the latter in December 1958 under the chairmanship of Dr. West, who also chose the topic of the symposium, selected the contributors, and edited the papers. This volume includes all the papers presented there and some of the discussions. There are twenty-five papers, eight of which report experiments with sensory isolation.