Optical imaging can be conducted on both the microscopic scale and the macroscopic scale. Microscopic examination can observe cellular structure and biomarkers, using absorptive and uorescent dyes and phase-sensitive techniques, and observe tissue architecture using optical coherence tomography. Practical clinical optical imaging, however, must still survey large tracts on tissue, on the scale of many millimeters or centimeters. Yet such macroscopic surveillance must remain sensitive to the microscale structure of the tissue in order to achieve meaningful contrast. Toward this surveillance task, several investigative groups are developing label-free imaging based on light scattering. While a pixel in a macroscopic image does not image the microscale, it can characterize the microscale of tissue within that pixel by the absorption, uorescence, and scattering behavior. In order to use label-free scattering as a contrast mechanism, an understanding of the relation between light scattering and tissue microscale structure, from the sub-nm to several μm scale, is being developed.