ABSTRACT: Functional materials such as those for optoelectronic and energy-related applications are often structurally and chemically complex, where defects and impurities can be vital or fatal to the materials' performance. A detailed understanding of defect physics in these materials is thus required for explaining, predicting, and optimizing their properties, and for designing materials with improved performance. With recent advances in electronic-structure methods, first-principles calculations for defects and impurities have become an important tool in providing such an understanding. In this talk, I focus my discussion on defect physics vis-à-vis functional properties in complex materials. Specific examples will be taken from recent work on rare-earth doped wide band-gap semiconductors for optoelectronic applications and complex oxides for metal-ion battery electrodes. Through these examples, I will illustrate how state-of-the-art defect calculations can serve as a study of materials' response to interventions–done on purpose and in a systematic and well-controlled manner–at the electronic and atomic level, and how such a study can provide a fundamental understanding of the materials and help uncover new science.