This second part presents a comprehensive overview of fundamental optical properties of the III Nitride Semiconductor. All optoelectronic applications based on III-nitrides are due to their unique optical properties and characterizations of III-nitrides. Much information, which is critical to the design and improvement of optoelectronic devices based on III-nitrides has been obtained in the last several years. This is the second of a two part Volume in the seriesOptoelectronic Properties of Semiconductors and Superlattices.