Abstract: Pressure probe techniques are basically employed to measure the size and changes in pressure in micro-systems. The pressure probe originally was designed to detect giant algal-cell turgor, then was adapted to measure turgor and other water-relation parameters of higher plants after being improved by reducing magnitude and increasing accuracy. The techniques are based on theories of the thermodynamics of liquid. Through calculation with use of equations from liquid physics, the techniques can be used to measure water flow and hydraulic conductivity through individual cells and tissues of plants, determine the relative flow of water and solutes over membranes and organs and their interactions, and determine the rigidity of the cell wall. The techniques now have become a diverse tool for research into plant physiology and eco-physiology. This pressure-probe technique can measure in situ permeability of cell membranes to water and solutes at the resolution of single cells and hence is a useful tool to study function and regulation of water channels (aquaporins) of intact plant cells. The recently developed xylem-pressure probe technique is the only means to directly measure negative pressure in xylem conduits. Additionally, the technique is used to extract sap from an individual cell and detect pH and ion concentrations in xylem conduits. In this paper, we introduce the basic principles and the theoretical background underlying the pressure probe and detail the operational skills.