The manipulation of the rate of atomic transport along and across interfaces (most notably grain boundaries) remains a central theme in the control of the microstructure and properties of inorganic materials. This is known as the field of interfacial kinetic engineering (IKE). In his opening Sosman Memorial lecture in 1973, Professor Kingery proposed a set of plausible concepts that he considered to be necessary and sufficient for the interpretation of ceramic grain-boundary phenomena, which provided a foundation for conducting IKE. This 2008 Sosman Memorial lecture provides both a retrospective assessment and a prospective viewpoint of this persistently challenging field of study. A major focus of this paper, like Kingery's Sosman lecture, is the demonstration of how new techniques of observation have led to conceptual advances. A noteworthy new concept that has emerged is that of grain-boundary complexions. Through control of the complexions, the interface transport rate and the material performance can be engineered toward a given application requirement.