Surface magnetic properties of transition metal thin films and multilayer structures can be drastically modified through the adsorption of adatoms or molecules forming hybrid interfaces. In this talk, I will discuss our theoretical work concerning i) the effect of a molecular overlayer on the magnetocrystalline anisotropy of Co films, and ii) the appearance of novel interfacial effects ascribed to electron correlation. By performing Density Functional Theory (DFT) calculations, we find that the easy axis of magnetization of a Co surface can be tuned along different directions, and even from in-plane to out-of-plane, when adsorbing different molecules . The mechanism is related to the formation of the chemical bond between the molecular C pz and the out-of-plane Co d orbitals. Ultimately, our results explain recent experiments reporting magnetic hardening at molecule-Co interfaces. Electron correlation effects are investigated by combining DFT and Dynamical Mean Field Theory, for layered systems, using a perturbative solver for the many-body problem [2, 3]. The results for adatoms and small molecules on Fe surfaces indicate that electron correlation is affected by the charge transfer between the adsorbate and the surface . This can be “engineered” in such a way to drive the surface towards intermediate and novel correlated regimes, which could otherwise not be found in any other material systems. Finally, we will show that similar concepts can also be extended to interfaces between magnetic 2d materials, such as Fe4GeTe2, and molecules.  A. Halder, S. Bhandary, D.D. O'Regan, S. Sanvito, and A. Droghetti, arXiv preprint arXiv:2302.07330.  A. Droghetti, M.M. Radonjić, A.Halder, I. Rungger, and L. Chioncel, Phys. Rev. B 105, 115129 (2022).  A. Droghetti, M.M. Radonjić, L. Chioncel, and I. Rungger, Phys. Rev. B 106, 075156 (2022).  D.M. Janas, et al., Adv. Mater 35, 2205698 (2023).