Coffinite (USiO4•nH2O) is a common mineral in uranium ores. It is often observed to replace uraninite during alteration under reducing conditions. However, it has proven difficult to synthesize coffinite in the laboratory and quantitative thermodynamic data for coffinite are lacking. Despite these experimental difficulties, there is ample evidence in nature that many uranium deposits have encountered conditions where formation of coffinite has been favoured over uraninite during postgenetic alteration events. Coffinite is also found as a primary mineral in sandstone uranium deposits. This review elucidates the spatial relation between coffinite and uraninite as seen on different scales with different analytical methods. Some further insight into the mechanism of uraninite alteration in natural, reducing, Si-rich environments is gained and some new arguments put forward, including the question of the effect of impurities and dopants, defects, and grain size. The replacement of uraninite by coffinite is discussed in terms of solid-fluid interaction.