  We review recent results on the nonlinear development of thermal instability
in the context of the turbulent atomic interstellar medium (ISM). First, we
pre- sent a brief summary of the linear theory, remarking that, in the atomic
ISM, the wave mode is stable at small scales. Next, we revisit the growth of
isolated entropy perturbations in initially unstable gas, as a function of the
ratio $\eta$ of the cooling to the dynamical crossing times. Third, we consider
the evolution of {\it velocity} perturbations. These correspond to the wave
mode, and are stable at moderate amplitudes and small scales, as confirmed
numerically. Fourth, we consider the behavior of magnetic pressure in turbulent
regimes. We propose that recent findings of a poor B-rho correlation at low rho
are due to the different B-rho scalings for the slow and fast modes of non-
linear MHD waves. This implies that, in fully turbulent regimes, the magnetic
field may not be a very efficient source of pressure, and that polytropic de-
scriptions of magnetic pressure are probably not adequate. Finally, we discuss
simulations of the ISM (and resolution issues) concerned with the possibility
of significant amounts of gas being in the ``lukewarm'' temperature range be-
tween the warm and cold stable phases. The mass fraction in this range in-
creases, and the phase segregation decreases, as smaller scales are considered.
We attribute this to the enhanced stability of moderate, adiabatic-like veloc-
ity fluctuations with $\eta \gg 1$, to the recycling of gas from the dense to
the diffuse phase by stellar energy injection, and to the magnetic field not
being strongly turbulent there, possibly providing additional stability. Final-
ly, we suggest that the lukewarm gas can be observationally distinguished
through simultaneous determination of two of its thermodynamic variables.
