By reviewing the results of our analyses based on statistical-mechanical theories, we point out that the entropic effect arising from the translational motion of water molecules is a principal driving force in a variety of self-assembling and ordering processes in biological systems such as protein folding, molecular recognition, and ordered aggregation of protein molecules. The great entropic loss for the biomolecules accompanying these processes is largely compensated by a great entropic gain of the water that is present in the system. The microscopic mechanisms of protein folding and denaturation, receptor-ligand binding, and amyloid-fibril formation are discussed in detail. We describe an effort to develop a unique method for predicting the native structure of a protein. The roles of NaCl and cosolute molecules are also briefly discussed.