The significance of the thermal stratification for the energy efficiency of small solar thermal hot water heat stores is pointed out. Exemplary the thermal stratification build-up with devices already marketed as well as with devices still in development has been investigated experimentally and theoretically, taking into account different realistic operation conditions. The methods (selective temperature measurement, non-invasive field measuring methods PIV and LIF, computational fluid dynamics (CFD)) suitable for the experimental and theoretical analysis of thermal stratification devices are introduced. Copyright (C) 2007 John Wiley & Sons, Ltd.