Summary: We present a continuum mechanics based derivation of Reissner’s equations for large displacements and finite strains of beams, where we restrict ourselves to the case of plane deformations of originally straight Bernoulli-Euler beams. For the latter case of extensible elastica, we succeed in attaching a continuum mechanics meaning to the stress resultants and to all of the generalized strains, which were originally introduced by Reissner at the beam-theory level. Our derivations thus circumvent the problem of needing to determine constitutive relations between stress resultants and generalized strains by physical experiments. Instead, constitutive relations at the stress-strain level can be utilized. Subsequently, this is exemplarily shown for a linear relation between Biot stress and Biot strain, which leads to linear constitutive relations at the beam-theory level, and for a linear relation between the second Piola-Kirchhoff stress and the Green strain, which gives nonlinear constitutive relations at the beam theory level. A simple inverse method for analytically constructing solutions of Reissner’s nonlinear relations is shortly pointed out in appendix I.