  A general model independent approach using the `off-shell Bethe Ansatz' is
presented to obtain an integral representation of generalized form factors. The
general techniques are applied to the quantum sine-Gordon model alias the
massive Thirring model. Exact expressions of all matrix elements are obtained
for several local operators. In particular soliton form factors of charge-less
operators as for example all higher currents are investigated. It turns out
that the various local operators correspond to specific scalar functions called
p-functions. The identification of the local operators is performed. In
particular the exact results are checked with Feynman graph expansion and full
agreement is found. Furthermore all eigenvalues of the infinitely many
conserved charges are calculated and the results agree with what is expected
from the classical case. Within the frame work of integrable quantum field
theories a general model independent `crossing' formula is derived. Furthermore
the `bound state intertwiners' are introduced and the bound state form factors
are investigated. The general results are again applied to the sine-Gordon
model. The integrations are performed and in particular for the lowest
breathers a simple formula for generalized form factors is obtained.
