Hadamard states for bosonic quantum field theory on globally hyperbolic spacetimes Quantum field theory on curved spacetimes is understood as a semiclassical approximation of some quantum theory of gravitation which models a quantum field under the influence of a classical gravitational field that is a curved spacetime. The most remarkable effect predicted by this approach is the creation of particles by the spacetime itself represented for instance by Hawkings evaporation of black holes or the Unruh effect. On the other hand these aspects already suggest that certain cornerstones of Minkowski quantum field theory more precisely a preferred vacuum state and consequently the concept of particles do not have sensible counterparts within a theory on general curved spacetimes. Likewise the implementation of covariance in the model has to be reconsidered as curved spacetimes usually lack any non trivial global symmetry. Whereas this latter issue has been resolved by introducing the paradigm of locally covariant quantum field theory (LCQFT) the absence of a reasonable concept for distinct vacuum and particle states on general curved spacetimes has become manifest even in the form of no go theorems. [...]