In article number 1704290, Byung Hyo Kim, Dokyoon Kim, Taeghwan Hyeon, and co-workers summarize the research on large-scale synthesis of uniform-sized metal oxide nanoparticles and their biomedical applications. Specifically, the underlying mechanism for the synthesis of uniform-sized nanoparticles, surface modification steps to endow hydrophobic nanoparticles with water dispersibility and biocompatibility, and various medical applications, such as bioimaging, drug delivery, and therapy, are presented.