Climate change is a serious threat to the biodiversity of natural habitats and food security because of the worldwide reduction of crop yields. In this context, studying plants' responses to abiotic stresses such as water deficit, high temperatures, or soil and water salinity, exacerbated by climate change, has become a priority topic. There is a general agreement that all plants, regardless of their level of tolerance, use the same conserved responses to abiotic stress – control of ion transport, synthesis of compatible solutes, and activation of enzymatic and non-enzymatic antioxidant systems, amongst others – although the efficiency and the relative contribution of those responses to stress tolerance may vary widely between species. In particular, the accumulation of the osmolyte proline (Pro) is generally considered a relevant plant stress tolerance mechanism. Indeed, many publications report significant increases in Pro contents in plants subjected to different stress treatments. However, there are examples in which the absolute Pro concentrations reached are too low to have any significant osmotic effect, and the possible role of other osmolytes has not been investigated. Also, in many cases, a direct implication of Pro in tolerance mechanisms has not been demonstrated. In this talk, some examples will be presented, based on our group's work over the last 20 years, showing that, for a large number of species, Pro can be considered a reliable marker of the level of stress affecting the plants. However, the correlation between Pro levels and the relative tolerance of related taxa can be negative, suggesting that Pro is not directly involved in tolerance. In any case, we have observed a large variability in Pro accumulation patterns in response to stress treatments. Therefore, any generalisation is risky, and the study of Pro functional role in abiotic stress tolerance mechanisms should be performed on a case-by-case basis.