Uniqueness theorems for an inverse problem in a doubly periodic structure (This paper is submitted for publication to Inverse Problems.) Habib AMMARI Centre de Mathematiques Appliquees - CNRS URA 756, Ecole Polytechnique, 91128 Palaiseau Cedex, France. Consider an electromagnetic plane wave incident on a biperiodic structure in $\RR^3$. The inverse problem is to determine the shape of the structure from the scattered field. In this paper, uniqueness theorems are proved by applying the uniqueness theorem of Cauchy-Kowalewska, by extending Isakov approach and using a result on local injectivity of maps between finite-dimensional spaces. The whole paper is available as a compressed Postscript file by internet procedure FTP anonymous on host barbes.polytechnique.fr ( 129.104.4.100) in the directory pub/RI/1995 under the name ammari_311.jan.ps.gz or by Xmosaic or any other www client via the CMAP www server "http://blanche.polytechnique.fr/"