In this paper we prove the existence and the stability of small-amplitude quasi-periodic solutions with Sobolev regularity, for the 1-dimensional forced Kirchoff equation with periodic boundary conditions. This is the first KAM result for a quasi-linear wave-type equation. The main difficulties are: (i) the presence of the highest order derivative in the nonlinearity which does not allow to apply the classical KAM scheme, (ii) the presence of double resonances, due to the double multiplicity of the eigenvalues of −∂ xx . The proof is based on a Nash-Moser scheme in Sobolev class. The main point concerns the invertibility of the linearized operator at any approximate solution and the proof of tame estimates for its inverse in high Sobolev norm. To this aim, we conjugate the linearized operator to a 2 × 2, time independent, block-diagonal operator. This is achieved by using changes of variables induced by diffeomorphisms of the torus, pseudo-differential operators and a KAM reducibility scheme in Sobolev class.We consider the Kirchoff equation in 1-dimension with periodic boundary conditionsOur aim is to prove the existence and the linear stability of small-amplitude quasi-periodic solutions with Sobolev regularity, for δ small enough and for ω in a suitable Cantor like set of parameters with asymptotically full Lebesgue measure.The Kirchoff equation has been introduced for the first time in 1876 by Kirchoff, in dimension 1, without forcing term and with Dirichlet boundary conditions, namelyto describe the transversal free vibrations of a clamped string in which the dependence of the tension on the deformation cannot be neglected. It is a quasi-linear PDE, namely the nonlinear part of the equation contains as many derivatives as the linear differential operator. The Cauchy problem for the Kirchoff equation (also in higher dimension) has been extensively studied, starting from the pioneering paper of Bernstein [9]. Both local and global existence results have been established for initial data in Sobolev and analytic class, see [2], [3], [23], [24], [32], [40], [41]. Concernig the existence of periodic solutions, Kirchoff himself observed that the equation (1.2) admits a sequence of normal modes, namely solutions of the form v(t, xUnder the presence of the forcing term f (t, x) the normal modes do not persist, since, expanding v(t, x) = j v j (t) sin(jx), f (t, x) = j f j (t) sin(jx), all the components v j (t) are coupled in the integral term T |∂ x v| 2 dx and the equation (1.2) is equivalent to the infinitely many coupled ODEs v ′′ j (t) + j 2 v j (t) 1 + k k 2 |v k (t)| 2 = f j (t) , j = 1, 2, . . . .The existence of periodic solutions for the Kirchoff equation, also in higher dimension, have been proved by Baldi in [4], both for Dirichlet boundary conditions (v = 0 on ∂Ω) and for periodic boundary conditions (Ω = T d ). This result is proven via Nash-Moser method and thanks to the special structure of the nonlinearity (it is diagonal in space), the linearized operator at any approximate solution can be inverted ...