  Over the last few years much attention has been given to the study of
modified gravity theories in order to find a more natural explanation for the
late time acceleration of the Universe. Nevertheless, a comparison of the
matter power spectrum predictions made by these theories with available data
has not yet been subjected to a detailed analysis. In the context of f(R)
theories of gravity we study the predicted power spectra using both a dynamical
systems approach for the background and solving for the matter perturbations
without using the quasi-static approximation, comparing the theoretical results
with several SDSS data. The importance of studying the first order perturbed
equations by assuming the correct background evolution and the relevance of the
initial conditions are also stressed. We determine the statistical significance
in relation to the observational data and demonstrate their conflict with
existing observations.
