In this work the electronic structure as well as magnetic and thermoelectric properties of alloy systems containing the 3d-transition metals manganese, iron and cobalt have been investigated by means of first-principles methods and Monte Carlo simulations. In addition, a new analysis of magnetic exchange interaction in nickel-manganese based alloys is carried out by decomposing the magnetic coupling constants into contributions associated with orbitalsymmetries.</br> The survey starts with the investigation of the binary alloys Fe1-xMnx, Fe1-xCox and Co1-xMnx. Interesting new insight into ground state properties and finite temperature magnetism of Fe1-xMnx is obtained from the combination of ab initio methods and Monte Carlo simulations. The evaluated Curie temperatures are in good agreement with experiment and predicted ground state configurations of magnetic moments match the state-of-the-art theory of these alloys. Results of the investigation of Fe1-xCox allow an excellent mapping onto the experimental phase diagram and contribute to the current discussion on the occurrence of superstructures in these systems. As there is only few theoretical work concerning Co1-xMnx up to now, the investigation of these alloys delivers valuable new information. The first modeling of the complex magnetic properties of Co1-xMnx on a first-principles basis has been carried out and additionally, the occurrence of an unexpected structural phase is predicted.</br> In the second step, a series of Heusler alloys based on the three transition metals Mn, Fe,Co and the main group elements Al and Si is carried out. In this part, the focus is on halfmetallic ferromagnetism of such systems. Almost all combinations of these elements turn out to be half-metallic and certain compositions are half-metallic antiferromagnets. The influence of disorder on the specific electronic structure and the magnetic properties is analyzed and it turns out that disorder does not necessarily destroy half-metallicity if it is restricted to certain sublattices. On the other hand, magnetism and Curie temperature can be strongly affected. Comparing the results to the investigation of corresponding binary alloys reveals a clear relation but also the outstanding position of Heusler alloys.</br> During the investigation of magnetic exchange interactions in nickel-manganese based Heusler alloys the decomposition of the interaction into contributions associated with electron states of certain symmetry allows to understand how the strength of the interaction arises from particular contributions. In addition the contribution of magnetism to the driving force of the martensitic transition of such systems is qualitatively understood because the analysis reveals a certain type of magneto-structural instability. This opens an entirely new view on martensitic transitions in magnetic alloys.</br> The last part of this work concerns the thermoelectric properties of half-metallic Heusler layers in contact with platinum leads. Here, focus is on the spin dependence of the Seebeck coefficient. It turns out that the half-metallic property of the Heusler systems significantly affects the contributions arising from the two spin channels. The results show the possibility to design thermally driven spin generators and contribute to the modern field of spincaloritronics.