
The BetheSalpeter equation restores exact elastic unitarity in the schannel by summing up an infinite set of chiral loops. We use this equation to show how a chiral expansion can be undertaken in the twoparticle irreducible amplitude and the propagators accomplishing exact elastic unitarity at any step. Renormalizability of the amplitudes can be achieved by allowing for an infinite set of counterterms as is the case in ordinary chiral perturbation theory. Crossing constraints can be imposed on the parameters to a given order. Within this framework, we calculate the leading and nexttoleading contributions to the elastic pi pi scattering amplitudes, for all isospin channels, and to the vector and scalar pion form factors in several renormalization schemes. A satisfactory description of amplitudes and form factors is obtained. Ln the latter case, Watson's theorem is automatically satisfied. From such studies we obtain a quite accurate determination of some of the ChPT SU(2)lowenergy parameters ((l) over bar (1)  (l) over bar (2) = 6.1(0.3)(+0.1) and (l) over bar (6) = 19.14 +/ 0,19). We also compare the twoloop piece of our amplitudes to recent twoloop calculations.
