This review deals exclusively with the theoretical task of interpreting the measurement of a thermal neutron absorption rate in terms of the neutron density that existed at the probe location prior to probe insertion. Successful accomplishment of this task is fairly obviously of considerable practical importance, since activation measurements of thermal fluxes are an important adjunct to many reactor experiments and are also frequently employed to obtain thermal flux maps in reactor cores and reflectors. Because of its importance, the problem has received extensive and varied (but usually piecemeal and semi-intuitive) theoretical attention over the past twenty years. Many experiments have been performed concurrently. The net result has been to perpetuate a lively interest in the problem, since comparisons of experiment with experiment, experiment with theory, and theory with theory have been ragged and controversial.1 It will be the purpose of this review to present an adequately precise and general statement of the problem and then attempt deductive comparisons of various theoretical attacks upon it. Attention will be focused primarily upon the task of obtaining operational descriptions of the diverse approximations explicit or implicit in the calculations considered. Throughout the review attention is persistently directed to an aspect of the problem that has received less than adequate attention so far, i.e., that of the effect of “flux-hardening” on the absorption rate. An attempt to obtain a qualitative estimate of the importance of this effect is presented, but the only conclusion drawn so far is that the effect is not obviously an ignorable one.