The hydrothermal diamond anvil cell (HDAC) has been used to place aqueous samples under conditions up to 2.5 GPa and 1000 °C. Optical microscopy, X-ray diffraction (XRD), X-ray fluorescence (XRF), X-ray absorption fine structure (XAFS), infrared absorption, luminescence, and Raman spectroscopy have been used to analyze samples in the HDAC. Equation of state (EOS), critical behavior, melting, phase transitions, reactions, ionic structure and speciation, and compositional changes have been observed and measured. Five examples of the application of the HDAC discussed here are: 1) EOS of water, 2) hydration states of montmorillonite, 3) silicate melts under hydrothermal conditions, 4) prevention of decrepitation of CO2-rich fluid inclusions, 5) structure and speciation of complexes. The simplicity of diamond anvil cells and the diverse modifications for their use with a wide range of analytical techniques is leading to many exciting new experimental approaches to problems in mineralogy, petrology, and geochemistry.