The remarkable electrical, optical, thermal, antimicrobial, and magnetic properties, as well as the non-toxicity and low cost establish copper oxides as crucial materials in a wide range of fields, including catalysis, photocatalysis, sensing, energy storage, biomedicine and spintronics. Cu 2 O and CuO are p-type semiconductors with relatively narrow band gap energies (2.0 to 2.2 eV for Cu 2 O and 1.2 to 1.9 eV for CuO), which makes them particularly interesting as catalysts in photoelectrochemical reduction reactions, such as the CO 2 reduction reaction (CO 2 RR) and the nitrogen reduction reaction (NRR). Furthermore, Cu 2 O and CuO are 3d transition metal oxides (TMOs) and as such have unique magnetic properties. While Cu 2 O has a diamagnetic (DM) and CuO an antiferromagnetic (AFM) groundstate, their surfaces exhibit different types of magnetism. For example, room temperature ferromagnetic (FM) properties of Cu 2 O microcrystals, 10 room temperature ferromagnetism in pure CuO nanoparticles and size effects on the magnetic properties of Cu 2 O and CuO nanoparticles have been reported experimentally. Although several studies identified different types of magnetism in copper oxide nanoparticles, the origin of the observed surface magnetism is still not fully understood. Experimental reports suggested that the wide spectrum of magnetic properties may originate from defects such as cation and/or oxygen vacancies. However, a theoretical study by Yu et al. revealed that some low-index Cu 2 O surfaces exhibit surface ferromagnetism without the presence of defects. According to their work, the origin of the surface ferromagnetism of Cu 2 O can be understood within the framework of Stoner's theory and may be due to the more pronounced 2p-3d hybridization of surface Cu and O atoms compared with atoms underneath. In addition to this study, Soon et described the surface structure of different terminated low-index Cu 2 O surfaces and calculated their surface energies as a function of the oxygen chemical potential. They found that the Cu 2 O(110) surface, terminated with both Cu and O atoms (Cu 2 O(110):Cu/O), and the Cu 2 O(111) surface, containing a surface Cu vacancy (Cu 2 O(111)-Cu cus ), are energetically most favorable. In the case of CuO, the stoichiometric CuO(111):Cu/O surface has been identified as the most stable CuO surface by Hu et al. and Mishra et al. For this particular stoichiometric CuO surface, the structural and magnetic properties were previously investigated. To our knowledge, there are no theoretical reports on the structural or magnetic properties of polar nonstoichiometric CuO surfaces. In general, only a few theoretical studies have investigated the relationship between surface structure and surface magnetism of copper oxides, and some questions remain to be addressed. For example, it is still not entirely clear why some terminations of the low-index Cu 2 O surfaces exhibit surface ferromagnetism while others do not, and how the surface magnetism relates to the surface struc-ture in general. Especially in the field of spintronics, it is of great interest to understand the magnetic properties of nanocrystalline magnetic semiconductors, such as nanosized copper oxides. Therefore, further insights into the surface structure and the associated magnetism of copper oxides are needed to improve their performance for many applications. In this systematic study, we present detailed insights into the structural and magnetic properties of low-index Cu 2 O and CuO surfaces, which allow for a direct comparison of differently terminated copper oxide surfaces. Furthermore, we suggest a possible explanation for the experimentally observed magnetism within the framework of a simplified model.