Microvascular networks of human basal cell carcinomas (Bcc) and surrounding skin were assessed with optical coherence angiography (ocA) in conjunction with photodynamic therapy (pDt). ocA images were collected and analyzed in 31 lesions pre-treatment, and immediately/24 hours/3-12 months posttreatment. Pre-treatment OCA enabled differentiation between prevalent subtypes of BCC (nodular and superficial) and nodular-with-necrotic-core BCC subtypes with a diagnostic accuracy of 78%; this can facilitate more accurate biopsy reducing sampling error and better therapy regimen selection. Post-treatment OCA images at 24 hours were 98% predictive of eventual outcome. Additional findings highlight the importance of pre-treatment necrotic core, vascular metrics associated with hypertrophic scar formation, and early microvascular changes necessary in both tumorous and peri-tumorous regions to ensure treatment success. Basal cell carcinoma (BCC) is the most common slow-growing locally invasive malignant epidermal skin tumor, with majority of the lesions localized on the head and neck 1. There are many different classification schemes for the BCC types and subtypes including nodular, superficial, infundibulocystic, fibroepithelial and the aggressive growth variants such as infiltrative, micronodular and morpheaform (sclerosing) plus some combinations 2,3. BCC treatment management starts with accurate diagnosis, including histopathology 4,5 and the development of reliable and non-invasive techniques for its diagnosis is crucial. Currently such non-invasive technologies have limited reliability and more development and validation work is needed 6. Dermatoscopy is a useful tool in the preoperative prediction of the BCC subtype 7. However, the evidence of its clinical diagnostic efficacy is somewhat limited and in ambiguous lesions, the BCC subtype has to be assessed histopathologically 8. Modern optical imaging modalities such as wide-field fluorescence imaging 9 , confocal microscopy 10 , optical coherence tomography (OCT) 11-13 , polarization-sensitive OCT 14,15 , OCT-based angiography (OCA) 16,17 and photoacoustic imaging 18 and their combinations 19,20 offer new possibilities for detection and visualization of human skin tumor structure and microvascular network in real time. OCT is particularly promising as it offers non-invasive structural and functional 3D imaging of biological tissues with high spatial resolution 11 , in particular for BCC 9,13,21. For the first time in this study, we used spectral domain OCT/OCA system to assess the after-treatment effects following photodynamic therapy (PDT) in patients with different BCC subtypes. PDT involves administration of photosensitizes which preferentially accumulate in malignant tissues, followed by local light activation at appropriate wavelength, producing oxygen free radicals and singlet oxygen 22. The generation of these reactive oxygen species subsequently causes tumor cell death by apoptosis, necrosis