III-nitride semiconductor materials find applications in solid state lighting, solar blind detectors, solar cells and are highly promising for the development of next generation high power/high frequency transistor devices. However, due to the unavailability of native substrates, nitride device structures are grown on mismatched substrates such as Si, SiC and sapphire. The large lattice and thermal mismatches in nitride epitaxial architecture therefore lead to the presence of a huge density of defects and dislocations. It causes several problems like poor device efficiency, current collapse, DC-to-RF dispersion and parallel conduction effects etc. A fundamental understanding of defects and the involved recombination mechanisms is therefore of paramount importance. Availability of this knowledge can help in the minimization of defects and dislocations in nitride epitaxial layers grown on foreign substrates. Spectroscopic characterization techniques can provide vital information related to such defects in a truly non-destructive manner. In this seminar, novel methodologies for the spectroscopic characterization of nitride heterostructures and their application in probing the spatial and spectral location of defects will be presented. In particular, the origin of yellow luminescence band in AlGaN/GaN heterostructure, and blue luminescence band in heavily Mg doped p-GaN epilayers will be discussed. In later part, spectroscopic measurements towards an unambiguous identification of 2-Dimensiuonal Electron Gas features in AlGaN/GaN HEMT structures will be discussed. I shall also try to present our latest work on the impact of Mg doping during initial growth of GaN buffer layer in AlGaN/GaN HEMT structures.