The coherent diffraction of neutrons from powder samples combined with the Rietveld refinement method has proven to be a versatile tool, providing insights into a variety of mineralogical crystal-chemical studies that require measurements at non-ambient conditions. The relatively low absorption which neutrons suffer, the ease in taking sample-environment devices onto experimental stations at neutron facilities, the coherent-neutron-scattering length contrasts between quasi-isoelectronic species and the independence of coherent-neutron-scattering power of the scattering vector, underlie the success in using this radiation in mineralogy. The literature reporting neutron powder-diffraction studies of minerals at non-ambient conditions has been reviewed and the results obtained over the last 20 years or so are briefly reported in order to provide an indication of the diversity of applications of the technique. A few applications have been selected for a more in-depth analysis of the results (i.e., micas); further applications are discussed in detail in the accompanying paper by Redfern (2002). The results reported are gathered into four sections, according to the experimental conditions used: high temperature (cation partitioning, phase transitions), low temperature (location of light atoms, hydrogen bonding, occupancy factors of quasi-empty sites), high pressure (hydrogen bonding, phase transitions, cation partitioning) and high pressure and temperature (cation partitioning).