Abstract. This paper is concerned with two-dimensional, linearly elastic, composite materials made by mixing two isotropic components. For given volume fractions and average strain, we establish explicit optimal upper and lower bounds on the effective energy quadratic form. There are two different approaches to this problem, one based on the "Hashin-Shtrikman variational principle" and the other on the "translation method". We implement both. The Hashin-Shtrikman principle applies only when the component materials are "well-ordered", i.e., when the smaller shear and bulk moduli belong to the same material. The translation method, however, requires no such hypothesis. As a consequence, our optimal bounds are valid even when the component materials are not well-ordered. Analogous results have previously been obtained by Gibianski and Cherkaev in the context of the plate equation. 0. Introduction. The macroscopic properties of a linearly elastic composite material are described by its tensor of effective moduli (Hooke's law) a*. This fourthorder tensor depends on the microgeometry of the mixture as well as on the elastic properties of the components. There is a large body of literature concerning the estimation of a* in terms of statistical information on the microstructure; see, e.g., [9,39,41].Recently a related but somewhat different question has received much attention: given a fixed collection of component materials, can one describe all composites o* achievable by mixing these components in prescribed volume fraction? Known as the "(7-closure problem", this question arises naturally from problems of structural optimization; see, e.g., [24,26,33]. A complete answer is available only in a few special cases; see, e.g., [13,25,27]. Much more is known about the analogous question