We performed polarization-resolved surface second harmonic generation (SHG) experiments on thin films of Collagen I and IV molecules, as well as conventional CID measurements. We found that Collagen IV presents little CD and no SHG optical activity, whereas Collagen I exhibits large chiroptical effects involving both one-electron and excitonic coupling mechanisms. We estimated that these chiral components enhance the SHG signal from fibrillar Collagen in biological tissues by typically a factor of 2. By comparing the distinct behaviors of collagens I and IV in SHG microscopy and in surface SHG experiments, we concluded that SHG microscopy is a sensitive probe of the micrometer-scale structural organization of Collagen in biological tissues.