  We study the dynamics of strings by means of a distribution function f(A, B,
x, t) defined on a 9+1D phase space, where A and B are the correlation vectors
of right- and left-moving waves. We derive a transport equation (an analogous
to Boltzmann transport equation for particles) that governs the evolution of
long strings with Nambu-Goto dynamics as well as reconnections taken into
account. We also derive a system of coupled transport equations (an analogous
to BBGKY hierarchy for particles) which can simultaneously describe long
strings \tilde{f}(A, B, x, t) as well as simple loops \mathring{f}(A, B, x, t)
made out of four correlation vectors. The formalism can be used to study
non-linear dynamics of fundamental strings, D-brane strings or field theory
strings. For example, the complicated semi-scaling behavior of cosmic strings
translates into a simple solution of the transport system at small energy
densities.
