A Near Infrared Spectral Tomography (NIRST) system has been developed and integrated into a commercial Digital Breast Tomosynthesis (DBT) scanner to allow structural and functional imaging of breast in vivo. The NIRST instrument uses an 8-wavelength continuous wave (CW) laserbased scanning source assembly and a 75-element silicon photodiode solidstate detector panel to produce dense spectral and spatial projection data from which spectrally constrained 3D tomographic images of tissue chromophores are produced. Integration of the optical imaging system into the DBT scanner allows direct co-registration of the optical and DBT imageswhile also facilitating the synergistic use of x-ray contrast as anatomical priors in optical image reconstruction. Currentlythe total scan time for a combined NIRST-DBT exam is ∼50s with data collection from 8 wavelengths in the optical scan requiring ∼42s to complete. The system was tested in breast simulating phantoms constructed using intralipid and blood in an agarose matrix with a 3 cm x 2 cm cylindrical inclusion at 1 cm depth from the surface. Diffuse image reconstruction of total hemoglobin (HbT) concentration resulted in accurate recovery of the lateral size and position of the inclusion to within 6% and 8%respectively. Use of DBT structural priors in the NIRST reconstruction process improved the quantitative accuracy of the HbT recoveryand led to linear changes in imaged versus actual contrastunderscoring the advantages of dual-modality optical imaging approaches. The quantitative accuracy of the system can be further improved with independent measurements of scattering properties through integration of frequency or time domain data.