This book presents a breakage stress measurement method and a comprehensive number of results for ZERODUR® with various surface conditions in practical use and for determining the fatigue dominating stress corrosion. A verification test confirms the validity of the lifetime prediction model. Breakage stress results are also given for a selection of optical glass types. To estimate the threshold breakage stresses for glass types for which these stresses cannot be measured, correlations with other physical quantities such as Knoop hardness and fracture toughness are investigated. Recommendations are given for improving strength in design and operation. A set of well-known applications of ZERODUR® in astronomy proves the suitability of glass and glass ceramic materials under mechanical stress loads.