  Recollements of triangulated categories may be seen as exact sequences of
such categories. Iterated recollements of triangulated categories are analogues
of geometric or topological stratifications and of composition series of
algebraic objects. We discuss the question of uniqueness of such a
stratification, up to ordering and derived equivalence, for derived module
categories. The main result is a positive answer in the form of a Jordan
H\"older theorem for derived module categories of hereditary artin algebras. We
also provide examples of derived simple rings.
