Ferromagnetic semiconductors such as Mn-doped GaAs are of interest for a variety of ‘spintronic’ devices which may exhibit improved efficiency, speed, and functionality. STM is a useful technique for studying ferromagnetism in these materials at the atomic scale. I will discuss our studies of Mn acceptors within the surface layer of a p-doped GaAs crystal. We start by sublimating Mn adatoms onto the GaAs (110) surface, prepared by cleavage in ultrahigh vacuum. A voltage pulse applied with the STM tip allows us to replace a Ga atom in the surface with the Mn atom, thus forming a single Mn acceptor. Tunneling spectroscopy reveals an acceptor resonance whose position within the GaAs bandgap is sensitive to the local electrostatic environment. For example, we find that charged As vacancies can be manipulated to shift the Mn resonance by up to ~200 meV, effectively acting as an atomic-scale gate electrode. Pairs of Mn acceptors can also be formed with controlled separation. These pairs order ferro- or antiferromagnetically, depending on their orientation with respect to the GaAs crystal lattice. This suggests that growth methods which favor Mn substitution along certain crystallographic directions may improve the ferromagnetism in Ga1-xMnxAs.