Einstein and Planck on mass-energy equivalence in 1905-06: a modern   perspective
  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. 
J.H. Field
physics.hist-ph