Abstract. The main results of this paper are existence theorems for traveling gravity and capillary gravity water waves in two dimensions, and capillary gravity water waves in three dimensions, for any periodic fundamental domain. This is a problem in bifurcation theory, yielding curves in the two dimensional case and bifurcation surfaces in the three dimensional case. In order to address the presence of resonances, the proof is based on a variational formulation and a topological argument, which is related to the resonant Lyapunov center theorem. 1. Introduction. Nonlinear periodic traveling waves on the free surface of an ideal fluid tend to form hexagonal patterns. This phenomenon is the focus of a number of recent papers on the subject of water waves, and it is the topic of the present article. In previous work, various approximations to the evolution equations for free surface waves are used, in particular the KP system by J. Hammack, N. Scheffner, and H. Segur [11], and J. Hammack, D. McCallister, N. Scheffner, and H. Segur [12], and alternatively with certain formal shallow water expansions of the Euler equations by P. Milewski and J.B. Keller [16]. A natural question is whether similar patterns can be shown to occur in solutions of the full Euler equations themselves. This is the focus of a series of papers by the present authors. In [18] and in [20] we report on hexagonal wave patterns and other phenomena in numerical computations of solutions, which are shown to satisfy spectral criteria for numerical convergence to solutions of Euler's equations. In the present article we describe rigorous existence results for periodic traveling wave solutions in free surfaces. The goal is to prove the existence of nontrivial traveling wave solutions to the water wave problem for gravity and capillary gravity waves in two and three dimensions. In two dimensions this is proven for both gravity and capillary gravity water waves, constituting a new and relatively straightforward approach to the theorems of T. Levi-Civita and D. Struik. In three dimensions we prove the existence of traveling capillary gravity water waves. However, the problem of gravity waves in three dimensions exhibits the phenomena of small divisors, and it remains open. The theorem that we prove is given below.