In this work, several aspects of molecular magnetism were investigated by experimental and theoretical methods. Synthetic works with carbazole-based Schiff base ligands led to dinuclear complexes involving Cu(II), Co(II), and Zn(II) ions. The Cu(II) complexes showed weak antiferromagnetic exchange interaction. In the case of Co(II), slow magnetic relaxation properties were found. Multiconfigurational calculations on Co(II) complexes with a wide variety of [N2O2] coordination environments gave insight into occuring spin-orbit coupling effects. Furthermore, linearly coordinated Co(II) complexes have been identified as potentially high-performing single-molecule magnets. The magnetic properties of lanthanide(III) complexes with the triaminoguanidine ligand system were calculated with the CASSCF/RASSI methodology. Simulations of the susceptibility data were used for the interpretation of hydrogen bond mediated exchange interactions. Several known oligonuclear systems, including Cu4O4 heterocubanes, hexanuclear Cu(II) wheels, and triaminoguanidine-based Cr(III) triangles, were reinvestigated with focus on the exchange interaction between the metal centers. Employing the broken symmetry DFT methodology, their magnetic properties were reinvestigated.