This thesis describes experimental studies of Rydberg atom pair interactions. A new technique is described that combines trapping of ultracold Cesium atoms with Rydberg tagging time-of-flight spectroscopy to attain a velocity resolution of 2.5 cm/s. Using this technique, observed molecular resonances in Cesium spectra are characterized as either dissociating photo-initiated collision processes or bound long range Rydberg atom-Rydberg atom molecules called macrodimers.