A quantitative theoretical investigation is made of the role of photo-ionization of the gas in the development of ionization currents in gases under uniform fields. Using published values of the relevant absorption coefficients and atomic cross-sections, the theory is applied to the case of air, and the results then compared with those previously obtained experimentally. It is shown that photo-ionization can lead to electrical breakdown only under certain restricted conditions, which relate the ionization coefficients and atomic cross-sections for photon-molecule interaction; these restrictions are such that there must be considerable penetration of the gas by ionizing photons, and in many cases a high proportion of all the photons thus reach the cathode. It is also found that the theoretical curve for the growth of current, obtained on the assumption that photo-ionization is the only operative secondary ionization process, has the same general form as the growth curve based on the other secondary processes, e.g. secondary emission from the cathode; there are, however, significant differences in detail. This quantitative investigation supports the view that photo-ionization does not play a predominant role as a secondary ionization process leading to the electrical breakdown of air at values of p $\times $ d $\sim $ 760 cm mm Hg in uniform fields.