In this talk, I will discuss some aspects of non-linear structure formation. The calculation of the bispectrum usually involves linear physics on large scales and non-linear (but Newtonian) physics on small scales. However, in the squeezed limit, the bispectrum (unlike the power spectrum) couples large scales with small scales. I will present the calculation of the non-linear relativistic corrections to the dynamics of dark matter perturbations. They are of the same order of magnitude, in the squeezed limit, than a primordial local non-Gaussianity signal. Such signal is expected to be within the accuracy of the next generation of LSS surveys.