N=2 string loops and spectral flow

Based on our previous studies of the BRST cohomology of the critical N=2 strings, we construct the loop measure and make explicit the role of the spectral flow at arbitrary genus and Chern class, in a holomorphic field basis. The spectral flow operator attributes to the existence of the hidden ''small'' N=4 superconformal symmetry which is non-linearly realized. We also discuss the symmetry properties of N=2 string amplitudes on locally-flat backgrounds. (author)