Many advanced materials today derive their function from precise control of point defects Through manipulation of point defects, the electrical, optical, mechanical, and other properties of materials can be systematically modified and optimized This feature paper illustrates the type of progress that has been made in understanding the often complex equilibria exhibited by many materials and shows, by use of examples, how this knowledge can be applied to tailor materials' properties through defect engineering Examples which highlight key points of defect engineering include dark conductivity of radiation detectors chemomechanics and nanoionics of solid oxide fuel cell materials, sensitivity of high frequency resonators for high-temperature mass measurements, and emission lifetime and luminescence intensity of phosphor materials