In nature, green polymers (natural polymers) in plants and animals always coexist with water. The characteristic features of polymers organized in nature are difficult to understand without water. Specific features of green polymers are characterised via interaction with water molecules which strongly interact with the hydrophilic group of polymers. Molecular motion of the main chain of polysaccharides, whether extracted from wood, fungi, seaweed, or bacteria, is considerably enhanced in the presence of water. Not only in crystalline polysaccharides but also amorphous lignin, the effect of water on molecular motion is clearly observed by various experimental techniques. When the molecular motion of green polymers is investigated in the presence of water, molecular rearrangement occurs by the introduction of water into the system, and the higher-order structure is rearranged during molecular movement by heating conditions. Phase transition behaviour of water molecules is also affected in the presence of hydrophilic polymers, such as polysaccharides. Molecular enhancement of water molecules and hydrophilic polymers cooperate with each other and phase transition behaviour of the above system also corresponds to the above motion. Even the first-order phase transition of water is affected in the presence of polysaccharides. When glass transition behaviour of the natural polymer-water system is investigated, it is important to take into consideration the fact that the structural change of both components has necessarily taken place. In this book, the molecular relaxation of green polymers, especially the thermodynamic concept of green polymers and the bound water, will be described in Chapter 2. Various techniques to measure the glass transition of green polymers in dry and wet conditions are explained in Chapter 3. Special attention is paid to sample handling for controlling water content. Glass transition behaviour of various polysaccharides and model compounds is described in Chapter 4. An explanation of lignin and its synthetic model polymers is also given in Chapter 5.