  Extremely Red Objects (EROs, R-K>5) constitute a heterogenous class of
extragalactic sources including high redshift elliptical galaxies, dusty
star-forming systems and heavily obscured AGNs. Hard X-ray observations provide
an unique and powerful tool to uncover obscured nuclear or star-forming
activity. We present the results of XMM-Newton observations of the largest
sample of EROs available to date (about 450 objects over a contiguous area of
700 arcmin^2). Five of the 46 hard X-ray selected sources brighter than
3x10^{-15} cgs in the 2-10 keV band, are associated with EROs. All of the X-ray
detected EROs show rather extreme X-ray-to-optical flux ratios, suggesting the
presence of highly obscured AGN activity. We also report on the X-ray stacking
analysis of spectroscopically identified EROs in the Chandra Deep Field South.
