  We review several ways of defining the statistics of charge that is
transmitted through an electric circuit, the so-called "counting statistics".
We first pursue two approaches that explain the origin of "negative
probabilities" that have been found earlier in this context. The first analysis
involves degrees of freedom of the electric circuit only. In the second
approach an idealized charge detector is introduced. We study then general
linear detectors and how their back action on the electric conductor affects
the statistics of transferred charge. Phase-coherent conductors in the presence
of an electromagnetic environment are studied within a Keldysh action approach.
