The earmark of the intermetallic compounds known as heavy-Fermion or heavy-electron materials is a low-temperature linear electronic specific-heat coefficient (γ) which shows an upturn on cooling to low temperatures, typically below 4 K (Fig. 1). It is clear from data on the gap anomaly of heavy-electron superconductors that this γ is of electronic origin and, further, that f-electrons must have something to do with it. Only the 4f and 5f elements, typically thought of as forming valence-fluctuation compounds (mainly Ce, Yb, U, and Np) seem to also form heavy-electron compounds. We list a number of these heavy electron compounds in Table 1.