The present invention relates to novel 6,7-dihydropyrazolo[1,5-a]pyrazin-4(5H)-one derivatives of Formula I as negative allosteric modulators (NAMs) of the metabotropic glutamate receptor subtype 2 (“mGluR2”). The invention is also directed to pharmaceutical compositions comprising such compounds, to processes for preparing such compounds and compositions, and to the use of such compounds and compositions for the prevention or treatment of disorders in which the mGluR2 subtype of metabotropic receptors is involved.