This chapter presents different aspects of the modeling of magnetic properties in monomeric open-shell actinide complexes. Those properties are closely related to their electronic structure, which is difficult to achieve since none of crystal-field effects, electron-electron repulsion nor spin-orbit interaction is predominant. The electronic structure should be analyzed within the intermediate coupling scheme, between on one hand the Russell-Saunders coupling scheme where the inter-eletronic repulsion is considered before spin-orbit, and, on the other hand, the j-j coupling scheme where one-electron wave-functions including spin-orbit are used to build the many-electron wave-function. Ab initio calculations on these complexes are challenging, and SO-CAS based methods are still the quantum chemistry tool of choice since they include a balanced description of the three effects. It is only by a close interplay between experimental data which are sparse for transuranide complexes due to radioactivity, numerical methods, and model Hamiltonians that one succeeds to unravel the electronic structure and magnetic properties of these complexes.