The field of nonlinear optics has witnessed a rapid growth into biology within the last decade, driven largely by the observations of large nonlinear optical signals from native tissue in biological microscopy. The signals generated from collagen and other biopolymer assemblies exhibit intricate polarization-dependences that are intimately connected to local structure and orientation. A key step in bridging the macroscopic polarization to microscopic structure is the development and testing of predictive models for describing the NLO properties of biopolymer assemblies. In this Letter, we review the strengths and limitations of approaches for experimentally accessing the molecular tensors of peptides, nucleotides, and saccharides, which serve as the fundamental building blocks of biopolymers. (C) 2008 Published by Elsevier B.V.