First we explain the importance of the investigation on the structure and properties of supercritical water in relation to the chemical evolution and the environmental and energy issues of the 21st century. Molecular interpretations are given to the temperature dependence of the density, dielectric constant, and viscosity, hydrogen-bonding structure, and dynamics of supercritical water. The three-dimensional network structure, characteristic of ambient water, is broken down in hot expanded water at temperatures higher than ～200℃ and densities lower than ～0.9 g cm^-3. The number of hydrogen bonds per molecule has been determined by the NMR method combined with computer simulation; it decreases from ～4 for ambient water to 1-2 for supercritical water at 400℃ and the critical density (～0.32 g cm^-3). The NMR rotational correlation time (τ_2R) for supercritical water at the medium densities is in the range of 50-70 fs, two orders of magnitude smaller than the ambient value (2 ps). Supercritical water is shown to be an alternative to hazardous organic solvents; there are being found new hydrothermal organic reactions without catalyst for the development of green chemistry.