This paper discusses the results of a research program that studied the influence of internal curing on the microstructure of mortars. Internal curing uses water-filled lightweight aggregates (LWAs) to supply additional water to the cement paste as it hydrates, thereby enabling an increase in the degree of hydration. The increased hydration can result in the densification of the microstructure. In particular, the densification occurs at the interfacial zone around the LWAs. The current work attempts to obtain a better understanding of the beneficial effects of internal curing on the basis of experimental observations of the microstructure and the nanostructure. The objective of this study was to examine the differences and the similarities that exist at both the microscale and the nanoscale of conventionally cured mortars and internally cured mortars. The specimens were tested at different ages to examine the influences of the internal curing over time. Water sorption, scanning electron microscopy, and scanning atomic force microscopy were used for this study. It was found that LWAs can be used for internal curing to provide a greater degree of hydration in a small region around the aggregate interface, which results in a microstructure that is more dense and more homogeneous and that contains less calcium hydroxide.