The main objective of the research discussed in this thesis was to evaluate the methods and materials used by builders in ancient Ostia to produce mortar for masonry structures. The work was conducted with an eye toward understanding why the ancient builders selected specific materials and employed specific techniques. The research design included scientific investigations of mortar samples from selected structures in Ostia, all dated from brick stamp evidence to the 2nd century CE. The methods employed for this study included thin section petrography, modal analysis via point counting, scanning electron microscopy, X-ray diffraction, and image analysis. Results concluded the samples comprised lime binder and primarily volcanic tuff aggregates derived from the Colli Albani and Monti Sabatini volcanic districts. The selection of these aggregates was integral to mortar performance, as they contained alteration products that would have facilitated pozzolanic reactions during the production phase, greatly improving long term mortar performance and durability. Three distinct types of mortar representing unique mix designs were identified based on the material properties of the mortar samples and the dominant aggregate types. Unique mixes may have been proprietary to specific groups of builders in Ostia, which had been identified by other researchers comparing the architectural features and construction styles of various structures. The results of material analyses showed that the builders from 2nd century Ostia were highly skilled, used sophisticated techniques to produce high quality mortar, and that separate groups of builders employed unique mix designs to achieve similar results.