The following paper is available by anonymous FTP or over the Web Lambda Definability with Sums via Grothendieck Logical Relations by Marcelo Fiore and Alex Simpson We introduce a notion of *Grothendieck logical relation* and use it to characterise the definability of morphisms in *stable* bicartesian closed categories by terms of the simply-typed lambda calculus with finite products and finite sums. Our techniques are based on concepts from topos theory, however our exposition is elementary. The paper is written in a style appropriate for the conference Typed Lambda-Calculi and Applications where it is to be presented in April. However, we briefly discuss the true categorical content of the paper, which will be further expanded upon in a full version of the paper (forthcoming). The paper is available over the Web: http://www.dcs.ed.ac.uk/~mf/TYPES/glr.{dvi,ps} http://www.dcs.ed.ac.uk/~als/Research/glr.ps.gz or by anonymous FTP: ftp://ftp.dcs.ed.ac.uk/pub/mf/TYPES/glr.{dvi,ps} ftp://ftp.dcs.ed.ac.uk/pub/als/Research/glr.ps.gz Best wishes for a happy New Year, Alex Simpson -- Alex Simpson, LFCS, Division of Informatics, University of Edinburgh Email: Alex.Simpson@dcs.ed.ac.uk Tel: +44 (0)131 650 5113 FTP: ftp.dcs.ed.ac.uk/pub/als Fax: +44 (0)131 667 7209 URL: http://www.dcs.ed.ac.uk/home/als