In recent times, functional "organic materials: molecular and polymers", being lightweight, low cost and their ease of production, have made inroads into many areas of devices formerly occupied by "inorganic materials". Molecular monolayers deposited on Si exhibiting electronic functionalities are expected to push the scaling limits of Si microelectronics down to a few nanometers. Thin films of molecular semiconductors and conducting polymers are widely being used in various devices, such as, light emitting diodes, optically pumped photodetectors, lasers, printed transistors, photorefractive applications, solar cells, biomedical application, gas sensors, etc. Recent advances show improved physical characteristics of molecular/polymer materials as active semiconductors, indicating a bright future for molecular and polymer devices. New concepts, innovative fabrication techniques and material manipulation at the nanoscale are bringing about much improved performances, indicating that organic materials will herald a new technological base with inexpensive devices in the near future. This book focuses on this theme with an attempt to provide updates on recent advances made in certain key areas of development of molecular and polymer devices.