Polymers, both natural and synthetic, play critical roles in various environmental and industrial processes. The behavior of polymers in aqueous environments is largely governed by their physicochemical characteristics, such as polarity, molecular structure, crystallinity, and cross-linking density. Water can act as a plasticizer or structural modifier depending on whether it interacts loosely or firmly with the polymer matrix, influencing properties such as swelling, solubility, permeability, and thermal transitions. Several experimental and analytical techniques—such as gravimetric analysis, FTIR (Fourier transform infrared), NMR (nuclear magnetic resonance) spectroscopy, differential scanning calorimetry (DSC), and advanced hybrid tools such as AFM-IR (atomic force microscopy-infrared spectroscopy)—have been employed to investigate the molecular mechanisms underlying polymer–water interactions. Similarly, the complex interplay of structural factors and environmental conditions determines how water interacts with polymer systems, which in turn affects their function, durability, and suitability for various applications. Therefore, the current chapter presents a comprehensive overview of the fundamental principles, classifications, influencing parameters, and experimental methodologies that govern water–polymer interactions.