Fullerenes are the third allotrope of carbon and have received much attention due to their unique properties. In this monograph on 'Optical Gain and Relaxation Mechanism of Fullerenes in Solutions', the effect of concentration of Fullerenes C60 and C70 molecules on their fluorescence spectra are studied in detail. Moreover, Forster's Resonance Energy Transfer (FRET) from different dyes to the fullerenes are investigated and it has been found that Fullerene C60 -Coumarin C440 pair and also Fullerene C70 - Quinizarine pair may be used as spectroscopic rulers. Time Resolved Spectra of both the fullerenes in pure as well as in mixture form were measured and the results show that the optical gain is concentration-dependent and only Fullerene C70 shows positive gain, which can be used as a lasing material.