Polybenzoxazine is a novel class of thermosetting material that belongs to the family of addition-curable phenolic resins. It is a promising high temperature polymer in view of its outstanding thermal, physical, and mechanical properties that are beneficial for their application in aerospace and microelectronics industries. The first objective of this book is to provide the reader with information on the developments and properties of contemporary thermosetting polymers, along with the chemistry and curing kinetics of polybenzoxazines. The unique properties of polybenzoxazines originate from their hydrogen-bonded structures. The second objective of this book is to furnish detailed information on the syntheses and properties of various polybenzoxazines prepared from different monomers. They are categorised mainly into two types; firstly, the benzoxazine monomers containing additional crosslinking groups such as maleimide, nitrile, propargyl, acetylene, furan etc. The other is based on monomers synthesised from different phenols and amines of varying backbone structures. The third objective of this book is to elaborate the preparation and properties of the modified polybenzoxazines such as their blends and nanocomposites. The final objective of this book is to provide specific information on the degradation chemistry of polybenzoxazines under thermal, photochemical and chemical environments. The potential areas of application of these polymer systems are highlighted. This volume is intended to be a quick reference to researchers from the academia and industries on polybenzoxazine science.