Optical microscopy has become an indispensable tool for visualizing sub-cellular structures and biological processes. However, scattering in biological tissues is a major obstacle that prevents high-resolution images from being obtained from deep regions of tissue. We review common techniques, such as multiphoton microscopy (MPM) and optical coherence microscopy (OCM), for diffraction limited imaging beyond an imaging depth of 0.5 mm. Novel implementations have been emerging in recent years giving higher imaging speed, deeper penetration, and better image quality. Focal modulation microscopy (FMM) is a novel method that combines confocal spatial filtering with focal modulation to reject out-of-focus background. FMM has demonstrated an imaging depth comparable to those of MPM and OCM, near-real-time image acquisition, and the capability for multiple contrast mechanisms.