Nowadays semiconductor ceramics represent an important field in the world of nanomaterials due to their electrical, optical, and mechanical properties, which make them useful in a high variety of applications. In the last decades, the research of semiconductor nanomaterials has been focused on the obtaining of intelligent materials for environmental applications such as photocatalysis for wastewater treatment and for energy applications such as photoanodes in solar cells, catalysts in hydrogen production, and materials up-conversion. Among these materials, metal oxides like TiO2 and ZnO are commonly used; however, they can only be activated with UV light because of their bandgap. To come across this problem, doped materials with elements like N, Fe, Cu, and some rare earths have been synthesized to modify the gap, obtaining materials that can be activated with visible light. However, the improvement of the efficiency of semiconductors with the introduction of an element in their structure relies not only on the bandgap modification but also on the modification of properties like crystallite size, morphology, particle size, and the creation of crystal defects. In the next lines, we will study how doping a nanomaterial affects its properties and improves its performance in some applications like photocatalysis, solar cells, hydrogen production, and materials up-conversion.