Abstract.A simple nonlinear beam model is derived from basic principles. The assumption upon which the derivation is based is that axial motions are of second order compared with transverse motions of the beam. The existence of solutions is established. Issues concerning the uniqueness and nonuniqueness of solutions are examined with regard to buckling behavior. The numerical treatment of problems with nonunique solutions is presented. The results of buckling calculations are presented. 1. Introduction. The objectives of this work are to (i) present in one place a physical derivation of a simple extension of the classic linear beam equation to a model which can capture nonlinear effects such as buckling, (ii) derive an associated variational principle that submits itself to analysis for existence, uniqueness, and nonuniquenss, and (iii) describe some of the numerical results in which nontrivial solutions are obtained. The study of elastic structures such as beams and plates is, to a large extent, dominated by linear models. Even the most familiar model used to predict the onset of buckling in elastic beams subject to longitudinal forces is a linear one. Nevertheless, it is well understood that incorporation of both "pre-buckling" and "post-buckling" phases into a single model requires the proposed model, at the least, to account for nonlinear geometric effects.There [13]. In this respect, our primary "point of departure" is to assume that "in line" or "in plane" displacements are of the same order as the square of the transverse and/or shearing displacements generally admitted into linear models of the same structures. The development of these models has been given new emphasis by recent developments in "formation theory," the deliberate modification of the configuration of an elastic body by means of attached or imbedded actuators [14]. We will not pursue such a study in this paper, leaving that work for another article. The objective here is to present a derivation of the model from first principles and to indicate how the model may be useful for the analysis of "buckling"-type bifurcation problems, not only with regard to prediction of "critical" loads initiating such bifurcation, already possible with the standard linear model, but also permitting studies of the progressive growth of the buckling phenomenon as the load is increased beyond the critical buckling load.In section 2, we derive a set of geometrically nonlinear equations for the elastic beam. These equations compare with those presented in [10]. Here, however, derivations, although ad hoc, are based on explicitly stated fundamental principles. Then, in section 3, we demonstrate existence and uniqueness of solutions from a variational