The Many Faces of Interpolation
Johan van Benthem

Abstract:
This is a survey of five unusual takes on what
the Interpolation Theorem says, as a form of
'model-crossing inference' involving changing
vocabulary, mixed with preservation behaviour.
Connections include Bolzano in the history of 
logic, and computational fixed-point logics. 
The material was presented at the Berkeley 
'Craigfest', May 2007, honouring Bill Craig's 
90th birthday, and it will appear in a special 
issue of "Synthese", edited by Paolo Mancosu.