In the context of collaborative dialogue, we analyze
conditional responses of the form ``Not (if) 
c/Yes if c'' in reply to a question under discussion ``q''. A
conditional response is used when the validity of "q"
depends on a condition "c": when "c" is established in the context,
the response indicates a possible need to revise "c", and thus
 opens negotiation; otherwise, the response  raises the question whether "c". 
We discuss appropriateness conditions for conditional responses, and propose a
uniform approach to their generation and interpretation.