  Form factor bootstrap approach is applied for diagonal scattering theories.
We consider the ADE theories and determine the functional equations satisfied
by the minimal two-particle form factors. We also determine the
parameterization of the singularities in two particle form factors. For
$A^{(1)}_{2}$ Affine Toda field theory which is the simplest non-self conjugate
theory, form factors are derived up to four-body and identification of operator
is done. Generalizing this identification to the $A^{(1)}_N$ Affine Toda cases,
we fix the two particle form factors. We also determine the additional pole
structure of form factors which comes from the double pole of the $S$-matrices
of the $A^{(1)}_N$ theory. For $A_N$ theories, existence of the conserved ${\bf
Z}_{N+1}$ charge leads to the division of the set of form factors into $N+1$
decoupled sectors.
