The ubiquity of smartphones offers a unique lens for studying human behavior in natural settings. Smartphone sensing, an expanding field driven by the widespread use of sensor-rich smartphones, facilitates observation and analysis of natural, “in-the-wild” behavior. These devices enable the seamless and unobtrusive collection of data across a broad spectrum of behavioral aspects, such as physical movement, social interactions, and smartphone usage patterns. This chapter provides an overview of smartphone sensing for understanding user behavior, outlining the types of sensors found in smartphones, such as motion and interactivity sensors, and their applications in capturing diverse aspects of user behavior. It also discusses the methodologies for data collection, distinguishing between passive and active sensing, and summarizes methods for data preparation and analysis in extracting meaningful insights from the raw sensor data. The practical applications of smartphone sensing are explored across multiple domains, including health monitoring, education, and social behavior analysis, demonstrating its versatility and potential to inform and enhance various aspects of daily life. The chapter also addresses the challenges inherent in conducting in-the-wild studies, such as ensuring data quality, maintaining participant engagement, and navigating technological constraints. Ethical considerations, particularly regarding privacy, informed consent, and data security, are highlighted as critical components of responsible research in this field. Looking ahead, the chapter identifies areas for future research, including the refinement of data collection and analysis methodologies and the ethical implications of advanced and developing sensing capabilities.