An attempt has been made to clarify the two most important issues relevant to personal eye dosimetry. This involves the identification of the cells which are most at risk from radiation and the specification of their position in the eye. A survey of the radiobiological literature concerning animals and humans shows that the epithelial cells in the equatorial region of the lens are those which are involved in radiation cataract induction. The depth of these cells has been evaluated in the human eye by means of geometrical construction. The relevant dimensions have been determined from a survey of published anatomical data and supplemented by new data obtained by slit-image photography. In a normal adult population (20-65 years) the minimum depth of the incriminated cells is 2.3 plus or minus 0.4 mm; the upper and lower values are associated with young and old subjects respectively. Approximate calculations for isotropic 90Sr/90Y and 106Rh beta-radiation fields indicate that a planar dosemeter, which integrates the tissue dose between depths of 2.5-3.5 mm, should give a reasonable measure of the mean equatorial dose for the variety of eye and irradiation geometries likely to be met during a life-time exposure. The long established, but tentative, value of 3 mm for the effective depth of the lens is thus confirmed.