I examine interactions between ensembles of cold Rydberg atoms, and between Rydberg atoms and an intense, optical standing wave. Because of their strong electrostatic interactions, Rydberg atoms are prime candidates for quantum information and quantum computation. To this end, I study excitation dynamics in many-body Rydberg systems using a rotary echo technique similar to the echo sequences used in nuclear magnetic resonance schemes. In this method, a phase reversal of a narrow-band excitation field is applied at a variable time during the excitation pulse. The visibility of the resulting echo signal reveals the degree of coherence of the excitation process. Rotary echoes are measured for several nD5/2 Rydberg levels of rubidium with principal quantum numbers near n = 43, where the strength of electrostatic Rydberg-atom interactions is sharply modulated by a Forster resonance The Rydberg-atom interactions diminish the echo visibility, in agreement with theoretical work. The equivalence of echo signals with spectroscopic data is also examined.;Applications of Rydberg atoms based on controlled interactions require a trapping device that holds the atoms at well-defined positions several microns apart. Rydberg atoms in ponderomotive optical lattices present a unique platform to meet this requirement, as well as to study properties and interactions of these highly excited atoms. Because the Rydberg electron is so loosely bound, the ponderomotive interaction for a Rydberg electron is very similar to a free electron. Ponderomotive lattices tailored to trap Rydberg atoms will allow new experiments in quantum information physics and high-precision spectroscopy. Microwave spectroscopy is used as a powerful technique to probe the motion and to verify trapping of Rydberg atoms in ponderomotive lattices. The potentials for non-degenerate, low angular momentum states, are used to obtain ensembles of Rydberg-atom trajectories in the lattice, and to simulate the spectra of microwave transitions of Rydberg atoms moving through the lattice. Additionally, adiabatic potentials are calculated for Rydberg atoms in one-dimensional ponderomotive lattices for a variety of atomic states and lattice parameters. The lattice induced mixing of nearly-degenerate, high-angular-momentum states is explained in terms of effective electric and magnetic fields.