Near-infrared mammography is a safe and painless breast imaging technique that is inherently sensitive to the hemoglobin concentration and oxygenation within the breast. The potential of this functional imaging technique for the detection of breast cancer has been explored for many decades with only partial success. The major limitation has been the qualitative nature of the oxygenation information provided by previous approaches to optical mammography. The broad objective of this application is to develop a new optical tool for functional breast imaging, which is aimed at discriminating benign and malignant breast lesions on the basis of a quantitative measurement of their oxygenation level. In the R21 Phase of this proposal, we intend to experimentally demonstrate the feasibility and the potential effectiveness of a novel spectral approach to optical mammography. To this aim, we plan to perform experimental tests on breast-like phantoms to simulate a quantitative measurement of the oxygen saturation of hemoglobin within breast lesions. The proposed approach is based on our quest for a robust oxygenation measurement of breast tumors that is insensitive to the high variability in the shape, size, and depth of the tumors. The basic hypothesis behind this proposal is that optical measurements at two optimal wavelengths, which are identified within the 680- 880 nm spectral band using a novel scheme of data analysis, can provide an accurate measurement of the tumor oxygenation. In the R33 Phase, we will implement the proposed spectral method into a practical instrument for breast imaging that is applicable to human subjects. This instrument will work in an optical transmission mode through the slightly compressed breast, and will measure two-dimensional projection images of the breast by means of a planar tandem scan of the illumination and collection optical fibers. Following preliminary tests of safety and technical performance, we will carry out a pilot study on human subjects to investigate the practical effectiveness of our proposed spectral approach. In particular, we will assess its potential in the detection and discrimination of benign and malignant breast tumors on the basis of their oxygenation levels. Our proposed approach to optical mammography has the potential to improve both the specificity and the sensitivity of current optical instruments, and it may open new opportunities in the area of breast cancer detection.