Following a discussion of existing probe theories for a collisional plasma, a new theory is given for ion collection by a spherical probe in a collisional plasma which goes over to the collisionless theory of Allenet al. at low pressures and the continuum theory of Su and Lam at high pressures. Numerical results are given in a form suitable for interpretation of experimental data. Experimental probe studies in the positive column (1 cm diameter) of a He discharge in the pressure range 0.05‐0.4 Torr are described and the use of the electron and ion saturation methods for determining plasma concentration compared with one another and with a microwave cavity perturbation method. The limitations of the theories and experimental methods are discussed.