  The review of possible stimulation mechanisms of LENR (low energy nuclear
reaction) is represented. We have concluded that transmutation of nuclei at low
energies and excess heat are possible in the framework of the modern physical
theory - the universal resonance synchronization principle [1] and based on its
different enhancement mechanisms of reaction rates are responsible for these
processes [2]. The excitation and ionization of atoms may play role as a
trigger for LENR. Superlow energy of external fields may stimulate LENR [3].
Investigation of this phenomenon requires knowledge of different branches of
science: nuclear and atomic physics, chemistry and electrochemistry, condensed
matter and solid state physics,... The results of this research field can
provide a new source of energy, substances and technologies.
  The puzzle of poor reproducibility of experimental data is due to the fact
that LENR occurs in open systems and it is extremely sensitive to parameters of
external fields and systems. Classical reproducibility principle should be
reconsidered for LENR experiments. Poor reproducibility and unexplained results
do not means that the experiment is wrong. Our main conclusions:
