Two essential questions are involved: (1) when to operate, and (2) what to do. An endeavor is made to find some means of evaluating and utilizing established operative procedures. It is felt that all methods, when properly selected and applied, must have their good points. With so many varied technics advocated, some differential basis must exist for selecting the method best adapted to the case at hand. Indications and contraindications for surgery are discussed. When nonoperative management has failed to show progress and the defect is becoming worse, with complications of amblyopia ex anopsia and progressive muscular dysfunction developing, surgery is indicated at any age. No one operation is ideal for all cases. A common theme is found connecting differential diagnosis and the surgery of muscular imbalance. Careful scrutiny makes possible a more exact classification of the nature of muscular anomalies. Such differentiation permits the election of various surgical procedures, using each to its best advantage. The relationship between differential diagnosis and the application of surgical corrective measures is discussed. It should be advantageous to be able to select from all the various operative technics the one best fitted to each case. A more refined diagnosis opens the way for such selection. From the Section on Ophthalmology, The Mayo Clinic. Read before the American Ophthalmological Society at Lucerne-in-Quebec, Canada, July, 1934.