  Einstein's theoretical analysis of mass-energy equivalence, already, at the
time, experimentally evident in radioactive decays, in two papers published in
1905, as well as Planck's introduction, in 1906, of the concepts of
relativistic momentum, and, by invoking Hamilton's Principle, relativistic
energy, are reviewed and discussed. Claims in the literature that Einstein's
analysis was flawed, lacked generality, or was not rigorous, are rebutted.
