An examination of the theory of the thermionic hollow cathode shows that several distinct modes of zero-fieldemission are possible. The mode that exists in any given case depends on the relationship between acertain parameterd, which characterizes the nature of the electron cloud within the cavity, and the size ofthe effusion hole and the cavity dimensions. A quantitative expression is given for the conditions underwhich the hollow cathode can be used for a precise determination of the thermionic constants by the methodof Jain and Krishnan. Under other conditions a quasi-hollow beam is obtained and a theoretical predictionof the total zero-field current from the hole was found to be in order of magnitude agreement with experimentalobservations. An explanation of the similarity of emissions between ‘L’ cathodes of different porosityis also given.