Recent progress in theory of nonlinear pulse propagation in optical fibers

We review recent achievements in theory of ultra-short optical pulses propagating in nonlinear fibers. The following problem is especially emphasized: what is the shortest duration (the highest peak power) of an optical soliton and which physical phenomenon is responsible for breakdown of too short pulses. We argue that there is an universal mechanism that destroys sub-cycle solitons even for the most favorable dispersion profile.