  The mutual conceptual incompatibility between GR and QM/QFT is generally seen
as the most essential motivation for the development of a theory of Quantum
Gravity (QG). It leads to the insight that, if gravity is a fundamental
interaction and QM is universally valid, the gravitational field will have to
be quantized, not at least because of the inconsistency of semi-classical
theories of gravity. If this means to quantize GR, its identification of the
gravitational field with the spacetime metric has to be taken into account. And
the resulting quantum theory has to be background-independent. This can not be
achieved by means of quantum field theoretical procedures. More sophisticated
strategies have to be applied. One of the basic requirements for such a
quantization strategy is that the resulting quantum theory has GR as a
classical limit. - However, should gravity not be a fundamental, but an
residual, emergent interaction, it could very well be an intrinsically
classical phenomenon. Should QM be nonetheless universally valid, we had to
assume a quantum substrate from which gravity would result as an emergent
classical phenomenon. And there would be no conflict with the arguments against
semi-classical theories, because there would be no gravity at all on the
substrate level. The gravitational field would not have any quantum properties,
and a quantization of GR would not lead to any fundamental theory. The
objective of a theory of 'QG' would instead be the identification of the
quantum substrate from which gravity results. - The paper tries to give an
overview over the main options for theory construction in the field of QG.
Because of the still unclear status of gravity and spacetime, it pleads for the
necessity of a plurality of conceptually different approaches to QG.
