Isotopes are useful probes of planet formation and evolution for at least two reasons. Some exhibit nucleosynthetic anomalies providing clues to their original location. Others undergo decay, providing age constraints. In this talk, I’ll discuss two applications of these ideas. The fact that the silicate Earth records different fractions of “carbonaceous” material in different elements implies that it experienced a change in the material it was accreting over time. The first ~95% was “non-carbonaceous”, while the last ~5% was “carbonaceous” and volatile-rich. Reconciling these constraints with existing planetary accretion scenarios is not easy. In the second part, I’ll discuss the idea that the pile-up of lunar ages around 4.35 Gyr is not the signature of magma ocean crystallization but is instead related to a tidal heating event. This story allows the Moon to be older (~4.45 Gyr or so), which is easier to reconcile with dynamical models.