  We formulate a general path integral approach which describes statistics of
current fluctuations in mesoscopic coherent conductors at arbitrary frequencies
and in the presence of interactions. Applying this approach to the
non-interacting case, we analyze the frequency dispersion of the third cumulant
of the current operator ${\cal S}_3$ at frequencies well below both the inverse
charge relaxation time and the inverse electron dwell time. This dispersion
turns out to be important in the frequency range comparable to applied
voltages. For comparatively transparent conductors it may lead to the sign
change of ${\cal S}_3$. We also analyze the behavior of the second cumulant of
the current operator ${\cal S}_2$ (current noise) in the presence of
electron-electron interactions. In a wide range of parameters we obtain
explicit universal dependencies of ${\cal S}_2$ on temperature, voltage and
frequency. We demonstrate that Coulomb interaction decreases the Nyquist noise.
In this case the interaction correction to the noise spectrum is governed by
the combination $\sum_nT_n(T_n-1)$, where $T_n$ is the transmission of the
$n$-th conducting mode. The effect of electron-electron interactions on the
shot noise is more complicated. At sufficiently large voltages we recover two
different interaction corrections entering with opposite signs. The net result
is proportional to $\sum_nT_n(T_n-1)(1-2T_n)$, i.e. Coulomb interaction
decreases the shot noise at low transmissions and increases it at high
transmissions.
