Nanotechnology involves the study at microcospic scale and has the potential to be the key to a brand new world in the field of construction and building materials. Although replication of natural systems is one of the most promising areas of this technology, scientists are still trying to grasp their astonishing complexities. This chapter covers the nanoscale analysis of Portland cement hydration products and the use of nanoparticles to increase the strength and durability of cementitious composites. It also covers the development of self-cleaning materials and materials that reduce air pollutants through photocatalysis.