  We discuss interacting quantum field theory in de Sitter space and argue that
the Mottola-Allen vacuum ambiguity is an artifact of free field theory. The
nature of the nonthermality of the MA-vacua is also clarified. We propose
analyticity of correlation functions as a fundamental requirement of quantum
field theory in curved spacetimes. In de Sitter space, this principle
determines the vacuum unambiguously and facilitates the systematic development
of perturbation theory.
