Water deserves a major attention by researchers dealing with biological systems and related materials, like food, since it is ubiquitous and can be used like a “native” probe to garner information about the hosting system, provided it may be freely displaced across. Its thermodynamic potential, namely, the water activity, aW, is related to that of the other compounds of the system considered via the Gibbs-Duhem relationship reflecting the extent of the residual availability of water to solvate further solutes and sustain the molecular mobility of the bio-polymeric compounds. As for the experimental approaches to aW, this short review re-addresses the reader to other publications, while devotes a section to the Knudsen thermo-gravimetry that was used by the authors to determine the desorption isotherms of many food systems and related aqueous compounds. The paper remarks the importance of a preliminary assessment of water mobility and recalls the concept of “critical aW “ that takes into account the reduced mobility of water molecules in the vicinity of the glass transition. This opens the question of the reliability of sorption isotherms which encompass a wide aW range and the interpretation of the observed adsorption/desorption hysteresis. The multi-phase character of many biological systems is another issue of interest related to the reliability of the experimental approaches to aW . As examples of the role of aW on the stability of bio-systems and on the practice of a technological treatment, protein unfolding and osmo-dehydration of fruit pulps are reported.