Polyimide (PI) is an engineering polymer material with advantages of high temperature resistance, low temperature tolerance, radiation resistance, flexibility and excellent dielectric properties. This book presents research on the properties, composites and uses of polyimides. Chapter One focuses on the synthesis and structure-property relationships of neat polyimides, and provides a list of the different methods available to improve the properties of neat PI for gas separation. Chapter Two presents a study aimed at clarifying the effect of fluorination time on surface charge and space charge behaviours of fluorinated PI film and PI/Al2O3 composites. Chapter Three focuses on the latest research and developments on PI aerogels, including crosslinkers synthesis chemistry, PI aerogels synthesis chemistry, and their engineering applications. Chapter Four summarizes the current fabrication methods of the polyimide aerogels and their composites.