  We study deformations of 2D Integrable Quantum Field Theories (IQFT) which
preserve integrability (the existence of infinitely many local integrals of
motion). The IQFT are understood as "effective field theories", with finite
ultraviolet cutoff. We show that for any such IQFT there are infinitely many
integrable deformations generated by scalar local fields $X_s$, which are in
one-to-one correspondence with the local integrals of motion; moreover, the
scalars $X_s$ are built from the components of the associated conserved
currents in a universal way. The first of these scalars, $X_1$, coincides with
the composite field $(T{\bar T})$ built from the components of the
energy-momentum tensor. The deformations of quantum field theories generated by
$X_1$ are "solvable" in a certain sense, even if the original theory is not
integrable. In a massive IQFT the deformations $X_s$ are identified with the
deformations of the corresponding factorizable S-matrix via the CDD factor. The
situation is illustrated by explicit construction of the form factors of the
operators $X_s$ in sine-Gordon theory. We also make some remarks on the problem
of UV completeness of such integrable deformations.
