The Pure Mathematics Department of Cambridge University has a new electronic preprint server (accessible via our home page at http://www.pmms.cam.ac.uk). The first preprint available may be of interest to people on the categories mailing list: it is C. Butz and P.T. Johnstone: Classifying toposes for first-order theories Abstract: By a classifying topos for a first-order theory $\Bbb T$, we mean a topos $\cal E$ such that, for any topos $\cal F$, models of $\Bbb T$ in $\cal F$ correspond exactly to open geometric morphisms ${\cal F} \rightarrow{\cal E}$. We show that not every (infinitary) first-order theory has a classifying topos in this sense, but we characterize those which do by an appropriate `smallness condition', and we show that every Grothendieck topos arises as the classifying topos of such a theory. We also show that every first-order theory has a conservative extension to one which possesses a classifying topos, and we obtain a Heyting-valued completeness theorem for infinitary first-order logic. For those who would prefer to receive a hard copy of this paper, I shall be bringing a supply with me to the Vancouver meeting. Peter Johnstone