  In this PhD thesis we deal with several theoretical and phenomenological
apsects of metric-affine theories of gravity. Concretely, we first give a broad
introduction to the necessary tools to understand the framework and elaborate
on some subtleties of the minimal coupling prescription between geometry and
matter in presence of torsion and nonmetricity. Then we dedicate the central
part of the thesis to study the structure of Ricci Based gravity (RBG)
theories, which will be of later use to understand generic properties of
metric-affine theories. We begin by analysing the structure of the RBG field
equations and nontrivial aspects of their solution space. We then analyse the
abrosption spectra of some spherically symmetric solutions. Then, we show that,
if the projective symmetry in these theories is explicitly broken, then there
arise ghost degrees of freedom, and we argue that this will be a generic
feature of metric-affine gravity theories. Having done this, we analyse
metricafine theories through the EFT lens, showing how the nonmetricity tkes a
particular form in generic theories where the symmetrised Ricci tensor appears
in the action beyond the Einstein-Hilbert term. This sources effective
interactions that we use to place tight constraints to these theories. In the
third part of the thesis we present a miscelanea of works which are not so
related to the structure of RBG theories. We begin by studying a model for
spontaneous breaking of Lorentz symmetry, namely the bumblebee model, in the
metric-affine approach. In the following chapter we generalise a conformal
invariant definition of proper time given by Perlick to the case with general
nonmetricity. Finally, we present arguments that show that the recently
proposed D4EGB theory is not well defined in its original form. We finish with
a brief outlook.
