This is a first announcement and call for participation for a Fields Institute Workshop on the interaction between category theory and semigroup theory. The workshop will take place at the University of Ottawa, May 2-4 2010, and everyone interested is welcome to attend. Topic, aims and scope: while the connection between inverse semigroups and ordered groupoids has been known for quite a while, it has only become clear fairly recently that many results and constructions in semigroup theory have natural and canonical topos-theoretic interpretations. The workshop aims to explore these connections, as well as those to locale theory, etale groupoids, categories of partial maps and also to operator algebras. The purpose is to bring together representatives from each of these areas, to further promote interaction and to develop new perspectives on the challenges in the field. There will be several tutorial talks which have the specific aim of bringing newcomers and non-experts up to speed on the central concepts, techniques and results of category theory and semigroup theory, and to introduce them to etale groupoids and operator algebras. Invited speakers: Robin Cockett (Calgary), Thierry Giordano (Ottawa) Mark Lawson (Heriot Watt), Ernie Manes (Massachussets), Dorette Pronk (Dalhousie), Pedro Resende (Lisboa) and John Rhodes (Berkeley). Student Support: graduate students are particularly encouraged to attend, and some funding is available in order to support their travel. Those who wish to apply for support can fill out the form on the workshop web page. Priority will be given to those students who do not have access to other funding. Registration: online registration is now open at: http://www.fields.utoronto.ca/programs/scientific/09-10/semigroups/index.htm... Accommodation: early May is a busy time in Ottawa and participants are urged to book sooner than later. We have reserved a block of rooms at the Quality Inn Hotel (10 min. walk from campus); please contact Pieter Hofstra for the booking code. [For admin and other information see: http://www.mta.ca/~cat-dist/ ]