In preparing the fifth edition of Plant Viruses the general plan of the book has been retained since this seems to offer adequate scope for a book of this size. However, each section has been brought up-to-date with any new information which has become available since the publication of the previous edition. Chapter 15 has been added giving a short account of two new fields of virus study, the viruses affecting fungi and algae. Some of the plates have been replaced by more modern illustrations. Reference to the aster yellows group of diseases has been eliminated because these diseases are now known to be caused by infection with Mycoplasma and not with viruses. Grateful acknowledgement is due to Dr Aaron Klug, F.R.S. and his colleagues for permission to use their recent work on the assembly of viruses. Acknowledgement is also due to several friends who have supplied prints of illustration from their pub lished work; credit has been given to authors in the illustration legends. Cambridge K.M.S.