Many microsurgical procedures are performed on the threshold of what is humanly possible. In the particular case of retinal surgery, these limitations are manifested in both the physical and sensory domains. Physical limitations include tremor, fatigue and positional accuracy, while sensory limitations include both tactile sensation and visualization. Addressing these limitations will make surgery faster, safer, cheaper and more effective while enabling new classes of procedures to be developed. During retinal surgery, instruments are placed within the eye and observed through the pupil using a stereo operating microscope. While visualization using this method is quite good, the optical properties of the cornea, lens and vitreous prevent retinal structures smaller than approximately 10 microns in size from being seen. To view objects smaller than this, imperfections in the optical path must be corrected using either mathematics or adaptive optics, or be circumvented by placing a high-resolution endoscope next to the retina. This paper concentrates on the latter of these options by describing the use of GRIN lens endoscopy for in-vivo microscopic observation and diagnosis during retinal surgery. The GRIN lens endoscope is capable of significantly greater resolving power than the operating microscope, therefore increasing visualization during surgery and likewise permitting surgical procedures not currently possible.