Taking advantage of rich molecular contrasts and safe non-ionizing radiation, optical imaging has been playing increasingly important roles in biomedical applications. However, a fundamental limit of optical imaging in biological tissue is light diffusion, which prohibits high-resolution imaging at depths beyond ~1mm. To break through this limit, Dr. Lidai Wang recently developed photoacoustic imaging and wavefront shaping technologies for in vivo functional imaging, early cancer detection, and focusing light into diffusive regimes. This presentation will first discuss the development of functional photoacoustic imaging modalities which, for the first time, achieved real-time quantitative imaging of oxygen release from single red blood cells in living tissue and enabled measurement of extremely slow blood flow in deep tissue. Then he will present a novel technique named nonlinear photoacoustic guided wavefront shaping (nonlinear-PAWS) that can effectively focus light to its diffraction limit in highly scattering media such as deep biological tissue.