To be cost effective, FRP decks are usually supported by steel girders.For the connection between FRP decks and steel girders, adhesive bonding technique is usually considered as a preferable connecting method, which can reduce construction time, save weight by eliminating fasteners, allow more uniform load transfer, achieve better adaption to the brittle and anisotropic nature of FRP materials and provide higher joint efficiency. Moisture dependent performance of FRP composites and adhesively-bonded joints are addressed in this book, regarding to 3D moisture diffusion analysis, mechanical degradation of FRP laminates under effects of moisture and temperature, and tensile-shear failure criterion of FRP-steel adhesively-bonded joints. Results of this research can contribute to the development of a design code of FRP-steel composite bridges. They can also be used as a reference information for understanding mechanical behaviours and durability of FRP-to-steel adhesively bonded joints for other applications in civil engineering field, such as strengthening of steel structures using FRP composite materials.