The physical basis and evidence in support of the cohesion-tension theory of the ascent of sap in plants are reviewed. The focus is on the recent discussion of challenges to the cohesion-tension mechanism based on measurements with the pressure probe. Limitations of pressure probes to measure tensions (negative pressures) in intact transpiring plants are critically assessed. The possible role of the cohesion-tension mechanism during the acquisition of water and solutes by plant roots is discussed.