This Volume of the Documenta Ophthalmoligica Proceeding Series presents the scientific papers read during the 13th Congress of SIDUO, the International Society for Ultrasonic Diagnosis in Ophthalmology, held in Vienna in the Summer of 1990. It was the second time that SIDUO had selected Vienna as a site for its biennial congresses in the 28 years of its existence. Previously, the 3rd SIDUO Meeting had taken place as part of the 1st World Congress for Ultrasonic Diagnosis in Medicine organized by Karl C. Ossoinig in 1969. Ossoinig, the pioneer of Standardized Echography opened the scientific sessions of SIDUO 13 with the First Jules Fran~ois Memorial Lecture on the Optic Nerve reviewing the modern examination techniques for precise measurements of the orbital optic nerve and its various sheaths and for an accurate diagnosis and differential diagnosis of diseases affecting the optic nerve, highlighting the new field of prophylactic recognition of early optic nerve compression (e. g. , in Graves' orbitopathy) and the important confirmation of CON in the presence of other conditions affecting the optic nerve functions. The first scientific session then dealt with a variety of interesting conditions of the optic nerve and of the extraocular muscles ranging from inflammatory diseases such as optic neuritis and orbital myositis to congenital anomalies, from glaucomatous changes to IIH, and from normal anatomical (dynamic) findings to malignant tumors such as lymphomas and leucemic meningeosis.