  In this article, we develop a theory of Grothendieck's six operations for
derived categories in \'etale cohomology of Artin stacks. We prove several
desired properties of the operations, including the base change theorem in
derived categories. This extends all previous theories on this subject,
including the recent one developed by Laszlo and Olsson, in which the
operations are subject to more assumptions and the base change isomorphism is
only constructed on the level of sheaves. Moreover, our theory works for higher
Artin stacks as well.
  Our method differs from all previous approaches, as we exploit the theory of
stable $\infty$-categories developed by Lurie. We enhance derived categories,
functors, and natural isomorphisms to the level of $\infty$-categories and
introduce $\infty$-categorical (co)homological descent. To handle the "homotopy
coherence", we apply the results of our previous article arXiv:1211.5294 and
develop several other $\infty$-categorical techniques.
