The higher the engine operating temperature, the higher its performance and efficiency. Nowadays, the operating temperatures are approaching the limits of Ni-based superalloys. Fortunately, niobium silicide based in situ composites have been shown to have great potential as alternative materials to Ni superalloys due to their high melting point, low density and excellent specific mechanical properties at high temperatures. This book begins with a brief review of the recent development of niobium silicide based in situ composites, including the related phase equilibria, processing, mechanical properties and oxidation behavior. The effects of Ti, Hf and/or Sn additions on the microstructures, solidification path, phase stability and oxidation kinetics of in situ composites based on the Nb-Si-Cr-Al-Mo system are discussed in details. This book is useful to understand the effects of alloying on microstructure development and phase selection in niobium silicide based in situ composites and to understand oxidation kinetics and mechanisms and their dependence on alloy chemistry and microstructure.