_Natural Behavior, Volume 66_ highlights new advances in the field, with this new volume presenting interesting chapters written by an international board of authors. There is a long history of studying natural behavior in science. In 1872, Charles Darwin documented his observations on the development of his children in words, which was published in an article titled “A Biographical Sketch of an Infant.” Traditionally, observational studies like this had been viewed as insightful but also criticized as not objective and quantitative. More recently, building on advanced computation, the contemporary approaches to studying natural behavior in the real world delivered quantitative results. New sensing and wearable technologies allow researchers to collect high-density data in everyday contexts. With technological advances, we can scale up and obtain quantitative results from real-world data. This volume contains a collection of papers on studying natural behavior of child development. Those papers aim at understanding and predicting behavior and cognition as it occurs within complex real-world situations. Compared with findings from laboratories, the results derived from natural behavior are remarkably reliable, which provides an answer to the reproducibility crisis in science. Moreover, the findings based on natural behavior can be directly applied to the real world, especially in the health and education domains. * Latest research on understanding development based on children’s natural behavior, rather than behavior based on short-term visits in laboratory settings * New methods for studying and analyzing children’s natural behavior across short and extended time scales * Cross-cutting research across different domains (e.g., language, cognition, interpersonal coordination), linked by a focus on natural behavior