Based on our developments of Spin-Polarized Scanning Tunnelling Microscopy (SP-STM) and Magnetic Exchange Force Microscopy (MExFM), both offering spin sensitivity and spatial resolution down to the ultimate limit of single atoms, we will study spin-dependent interactions between individual magnetic atoms on metal surfaces, in diluted magnetic semiconductors, on surfaces of magnetic insulators, as well as between single-atom tips and ultracold quantum gases. Besides the investigation of static spin states and spin interactions, we will manipulate spin states in a controlled manner down to the single atom limit by making use of the spin-transfer torque exerted by spin-currents from an atomically sharp SP-STM tip across a vacuum barrier. Moreover, we will combine spin-current induced magnetization switching experiments on magnetic metallic nanostructures based on SP-STM with pump-probe experiments, thereby studying the fundamentals of magnetization reversal processes both spatially and time-resolved. We will make use of the powerful combination of SP-STM with single-atom manipulation to probe spin-dependent interactions in artificial nanostructures. In the case of magnetic insulators we will probe spin states and spin-dependent interactions based on local measurements of the quantum-mechanical exchange and correlation forces between a single-atom tip with a well-defined spin state and single atoms of the sample. Spin excitations at the level of individual atoms will be probed by a combination of SP-STM with inelastic electron tunnelling spectroscopy, while the combination of MExFM with measurements of the damping of the cantilever oscillation will be employed to reveal local spin excitations in electrically insulating materials. Finally, we will couple an MExFM-type force sensor to the spin state of an optically trapped ultracold quantum gas with the challenging goal to combine scanning probe and quantum optical methods for manipulating quantum states of matter.