Hydrogeology, the study of groundwater storage, distribution and movement, faces increasing complexity due to climate change and human activities. Time series analysis has emerged as a cornerstone for understanding dynamic hydrological systems, enabling predictions critical for sustainable water management. This chapter reviews the role of time series data in hydrogeology. It first introduces the basics of time series hydrogeological data, then focuses on common analytical methods, detailed prediction models and real-world applications, finally summarizes the current challenges, and discusses the future development directions. By synthesizing classical and AI-driven approaches, this chapter aims to provide a comprehensive guide for researchers and practitioners in time series analysis and forecast in hydrogeology.