In this dictionary the authors' aim is to explain biological terms which a layman may meet when reading scientific literature; to define the terms which a student of biology has to master at the beginning of his career the thousand or so words which so grimly guard the approaches to the science and to provide a reminder for the professional biologist reading outside his own narrow field. The entries are not restricted to a bare definition: some information about most of the things named is given, so as to convey something of their significance in biological discussion. The authors have tried, as it were, to interpret a foreign language as it is actually used. It would be wrong to rely on etymology as a guide to correct usage. The meaning of a Greek root may be unequivocal, but biologists are not talking Greek: they are using a living language, and the proof of the meaning is in the speaking.