Biomicroscopy of the fundus during retinal detachment surgery has the same advantages as indirect ophthalmoscopy. In addition to this, it offers three main advantages over indirect ophthalmoscopy: the observation of the fundus, in an optical cross section of the tissues, the high variation of magnification available, and the observation of the fundus without switching off the light of the operating room. Since 1976--biomicroscopy of the fundus during retinal detachment surgery has been easily performed with the surgical mobile slit lamp manufactured by the firm Zeiss. The angle of the surgical slit lamp can be reduced to 5 degrees. The slit lamp can be used either with the Zeiss OPMI 1 operating microscope or with the Zeiss OPMI 6 operating microscope. The microscope, the contact lens and the surgical instrumentation specialy fitted for biomicroscopy of the fundus during retinal detachment surgery are described. The surgical technique is dealt with, pointing out the practical details which allow for overcoming the own difficulties of biomicroscopy of the fundus during surgery.