Historical development of the Bhabha first‐order relativistic wave equations for arbitrary spin

From letters, personal reminiscences, and the published works of the authors involved, we describe the historical development of the Bhabha system of first‐order relativistic wave equations for arbitrary spin. The simplest special cases of these equations reduce to the Dirac and DKP (Duffin–Kemmer–Petiau) equations for spin‐1/2 and spins 0 and 1, respectively.