The modern society is a so-called era of big data. Whereas nearly everybody recognizes the “era of big data”, no one can exactly define how big the data is a “big data”. The reason for the ambiguity of the term big data mainly arises from the widespread of using that term. Along the widespread application of the digital technology in the everyday life, a large amount of data is generated every second in relation with every human behavior (i.e., measuring body movements through sensors, texts sent and received via social networking services). In addition, nonhuman data such as weather and Global Positioning System signals has been cumulated and analyzed in perspectives of big data (Kan et al. in Int J Environ Res Public Health 15(4), 2018 ). The big data has also influenced the medical science, which includes the field of psychiatry (Monteith et al. in Int J Bipolar Disord 3(1):21, 2015 ). In this chapter, we first introduce the definition of the term “big data”. Then, we discuss researches which apply big data to solve problems in the clinical practice of psychiatry.