  We survey the landscape of $f(R)$ theories of gravity in their various
formulations, which have been used to model the cosmic acceleration as
alternatives to dark energy and dark matter. Besides, we take into account the
problem of gravitational waves in such theories. We discuss some successes of
$f(R)$-gravity (where $f(R)$ is a generic function of Ricci scalar $R$),
theoretical and experimental challenges that they face in order to satisfy
minimal criteria for viability.
