A 3-dimensional optical coherence tomography (OCT) system based on a high-speed micromachined scanning mirror is presented. The sampling resolution of each voxel is 10 mu m x 10 mu m x 10 mu m; the optical resolution is 20 mu m x 20 mu m x 10 mu m. In this work test-structures as well as in-vitro and in-vivo biological tissue samples are imaged. Cross-sectional images are collected and displayed in realtime; these images are subsequently combined to reconstruct a 3D image of the sample. Images from the 3D MEMS OCT system have been utilized to clearly identify dysplasia (initial stage of cancer) in hamster cheek pouch tissue by experts in the field, and have shown excellent correlation with histology slides of the same sample.