Some interrelated aspects of lead isotope systematics are exposed and their bearing on the problem of planetary evolution is discussed. In particular, three concepts are stressed: 1.(1) Point of isometry (PI): Pb-Pb isochrons of rocks of different ages which remain undisturbed since they were derived from the same homogeneous source intersect with each other at a single point (point of isometry) which describes the present day Pb composition of their common source. For a continually differentiating planet, where the composition of a source changes with time, a ‘domain’ rather than a ‘point’ of isometry may exist. However, because of geometric effects an illusionary point of isometry yielding a lower limit to the age of ‘the’ source—may still be obtained.2.(2) Dispersive isochronism: Isochronous, cognate, closed systems (e.g. an undisturbed rock and its minerals) that contain variable Th/U do not define an isochron on 208,206,204pb correlation diagram. The dispersion of the isotopic data of such systems from a line is proportional to the variability in Th/U. It is suggested that the individual dispersion trajectories may be experimentally determined. In theory these trajectories must all meet in a point of convergence (PC) which represents a true end point common to the systems. PC yields the composition of initial Pb which can be used to calculate the age of the source irrespective of one's success or failure in directly determining initial Pb from U and Th-poor mineral phases. Some experimental evidence to the feasibility of the method is provided.3.(3) Congruency of galenas: It is possible to construct a hypothetical ‘source profile’ which shows a spectrum of Pb compositions for sources of different ages which could have each produced a given galena at hand. Galenas of different ages, derived from the same homogeneous source, have different source profiles which nevertheless intersect each other at a single point corresponding to their parental source. Establishment of a point of intersection for a number of galenas is not an unequivocal proof for a homogeneous source, however, because mild heterogeneities could be smoothed out by geometric effects. Old conformable galenas yield an intersection corresponding to a common source which is 4.54 Byr old.