Advances in Solar Energy in its fourth year has almost become routine in identifying important fields that warrant comprehensive reports, in assembling its contents and in preparing the typeset version; the final r~sult is now in front of you for your judgement. In working with the authors to prepare attractive reviews, several of our referees and editors have helped with a great deal of their time and a wealth of suggestions and advice for further improvement. The subjects treated in the first four volumes covered many areas of the large field of solar energy conversion. The interested reader may anticipate missing subjects for following volumes or updates of earlier reviews in rapidly developing fields. As in earlier volumes, we invite your comments and suggestions for articles and authors who are eminently qualified to write such critical reviews. This Volume covers subjects in bioconversion, photovoltaics and, in three articles, subjects related to heat transfer to the ground. We hope to order the content of a Volume even more in the future, to give emphasis to a specific topic in solar energy conversion. We thereby try to indicate its different modes of application and to stimulate cross fertilization of related fields. My special thanks go to Ms. Sandra Pruitt and Ms. Patricia Porter-Revels for typesetting the manuscript in the Delaware Office, and to the University of Delaware for its support of the publications office. The accommodating help from Plenum Press and its production staff deserves our grateful acknowledgement.
