Optical microscopy, providing valuable biomedical insights at the cellular and organelle levels, has been widely recognized as an enabling technology. Mainstream optical microscopy technologies, including single-/multi-photon fluorescence microscopy and OCT, have demonstrated extraordinary sensitivities to fluorescence and optical scattering contrasts, respectively. However, the optical absorption contrast of biological tissues, which encodes essential physiological/pathological information, has not yet been fully assessable. The emergence of biomedical photoacoustics has led to a new branch of optical microscopy--OR-PAM. As a valuable complement to existing optical microscopy technologies, OR-PAM detects optical absorption contrasts with exquisite sensitivity: i.e., 100%). Combining OR-PAM with fluorescence microscopy or optical-scattering-based OCT: or both) provides comprehensive optical properties of biological tissues. Moreover, OR-PAM encodes optical absorption into acoustic waves, in contrast to the pure optical processes in fluorescence microscopy and OCT, and thus provides background-free detection. The acoustic detection in OR-PAM mitigates the impacts of optical scattering on signal degradation and naturally eliminates possible interferences: i.e., crosstalks) between excitation and detection, which is a common problem in fluorescence microscopy due to the overlap between the excitation and fluorescence spectra and imperfect extinction of the filter. Unique for high-resolution imaging of optical absorption, OR-PAM has demonstrated broad biomedical applications in fields such as neurology, ophthalmology, vascular biology, and dermatology. My doctoral research focuses on developments and biomedical applications of OR-PAM. The first part of my dissertation discusses the development of three generations of OR-PAM towards high-resolution, high-sensitivity, high-speed, and wide FOV in vivo imaging. In this section, I provide a comprehensive description of OR-PAM, including the principle, system design, ...