The thermomechanical stability of most commodity and engineering polymer films is poor under high temperatures and thus resulting in compromised mechanical properties and phase transformation or even decomposition of the polymers. By contrast, aromatic polyimides have been utilized in various industrial applications due to their high chemical/radiation resistance, thermal stability, strong mechanical strength, dimensional stability, low water absorption, tunable dielectric properties, and solution processability. These characteristics make polyimide suitable for serving as a substrate for flexible print circuit boards, organic light-emitting diode displays, coating films for very large-scale integration chips, dielectric layers in electronic devices, etc. With the great importance of developing high-performance polyimides for a plethora of applications, in this chapter, the recent developments of polyimides regarding their interrelationship between the monomer structures, polymer conformations, and thermal, mechanical, optical, dielectric, and electronic properties are reviewed. The applications of polyimides in high-temperature/colorless plastics, high-k dielectrics, low-k insulating materials, photoresists, and optoelectronic devices are highlighted in this chapter.