Difference schemes constitute important building blocks in the model-theoretic study of difference fields.
Our goal is to pursue their number-theoretic aspects much further than required by model theory.
Roughly speaking, a difference scheme (variety) is a scheme
(variety) with a distinguished endomorphism. We will explain how to extend the methods of etale cohomology to this context and, time permitting, we will show the calculation of difference etale cohomology in some interesting cases.