The miniaturization of electronic components such as transistors to nanoscale dimensions is nearing a fundamental limit where the discrete atomic nature of the dopants in semiconductor materials becomes important. In addition to scaling of conventional technologies, electro/optic control over single dopants is a basis for next-generation quantum-based information processing in the solid state. In this context, I will discuss our STM studies of impurity atoms in III-V semiconductors, GaAs and InSb. In our studies of p-doped GaAs, we introduce surface-layer acceptors (Mn, Co, Er) by first sublimating adatoms onto the 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 metal atom, thus forming a single acceptor and a Ga adatom. We find that the properties of acceptors can be tuned by control of the local electrostatic landscape, via (i) proximity to charged defects, (ii) control over the tunneling junction, or (iii) optical excitation. For example, the STM tip can be used to position As vacancies and other adatoms (e.g. Mn, Ga), all of which are positively charged. Direct Coulomb repulsion causes a reduction in the acceptor binding energy as these species are moved nearby. Tunneling spectroscopy allows us to quantify this effect, through the shift of an in-gap acceptor resonance toward lower energy. We have also discovered tip-induced ionization of Mn acceptors in GaAs, and adatoms on InSb, evident as a ring-shaped feature in STM imaging. While these ionization rings are subtle features in GaAs, we find they dramatically affect the conductance of the InSb surface, likely due to the smaller bandgap and shallow donor binding energy. We are also developing optical techniques that allow us greater insight into the properties of individual impurity atoms, and new opportunities for the control of those properties. We find that separation of photoexcited carriers probes the local electrostatic landscape and can be directly detected as a surface photovoltage with STM. Photoexcitation shifts the electronic states of impurity atoms, and also affects the tip induced ionization rings.