Most final diagnoses of cancer today are still made using the same histopathological techniques that have been in use for the past 50 years. In-vivo optical cancer detection (""""""""optical biopsy"""""""") holds tremendous potential to radically alter cancer screening, diagnosis and management. Over 15 years of study has shown that optical spectra reveal differences in normal and abnormal tissues. This proposal addresses the need for practical, cost-effective, and commercially-viable fiberoptic probes that can be used to detect a wide variety of cancers in-vivo. Principal targets are epithelial carcinomas of the lungs, skin, esophagus, GI tract, cervix, and bladder. The proposed probes use UV-visible polarized elastic scattering spectroscopy (PESS) to deduce differences in cellular microstructure that indicate pre-malignant or malignant growth. The development of probes that measure the spectra of scattered light in two polarization states will enable reductions in background signals, leading to higher signal-to-noise and improved sensitivity and specificity. State-of-the-art polarization technologies will be applied to the probe design. A quantity of probes will be fabricated and tested using tissue phantoms to establish optical improvements over non-polarized probes and establish feasibility for development of commercially-viable probes in Phase II. Phase II activities will include design refinement of the probes, fabrication scale-up, initial human trials, initial development of software algorithms for spectral data analysis, and establishment of commercial partners for manufacture and distribution. The successful implementation of probes for optical biopsy will allow physicians to perform biopsies at more sites, either intra-surgically or on a screening basis and can help guide surgical biopsy for improved sensitivity. Results of biopsies taken with these user-friendly probes will be immediate. They will allow the practitioner to perform additional measurements if needed, and minimize patient stress while waiting for results. Once optical biopsies are proven clinically, laboratory errors associated with sample handling and human fatigue will be completely eliminated. Screening rates should go up, and cancer diagnosis and treatment costs down. More than half of the 2 million cancer cases in the US each year involve carcinomas accessible to fiberoptic probes. If the probes can be further miniaturized for needle mediated access, additional cancers (breast, prostate) may be accessible.