  In this thesis, we wish to examine the black-hole solutions of modified
gravity theories inspired by String Theory or Cosmology. Namely, these
modifications will take the guise of additional gauge and scalar fields for the
so-called Einstein-Maxwell-Dilaton theories with an exponential Liouville
potential; and of extra spatial dimensions for Einstein-Gauss-Bonnet theories.
The black-hole solutions of EMD theories as well as their integrability are
reviewed. One of the main results is that a master equation is obtained in the
case of planar horizon topology, which allows to completely integrate the
problem for s special relationship between the couplings. We also classify
existing solutions. We move on to the study of Gauss-Bonnet black holes,
focusing on the six-dimensional case. It is found that the Gauss-Bonnet
coupling exposes the Weyl tensor of the horizon to the dynamics, severely
restricting the Einstein spaces admissible and effectively lifting some of the
degeneracy on the horizon topology. We then turn to the study of the
thermodynamic properties of black holes, in General Relativity as well as in
EMD theories. For the latter, phase transitions may be found in the canonical
ensemble, which resemble the phase transitions for Reissner-Nordstr\"om black
holes. Generically, we find that the thermodynamic properties (stability, order
of phase transitions) depend crucially on the values of the EMD coupling
constants. Finally, we interpret our planar EMD solutions holographically as
Infra-Red geometries through the AdS/CFT correspondence, taking into account
various validity constraints. We also compute AC and DC conductivities as
applications to Condensed Matter Systems, and find some properties
characteristic of strange metal behaviour.
