This book describes a theoretical study involving endohedrally doped fullerenes and carbon nanotubes and substitutionally doped fullerenes using the first principle methods based on density functional theory. The effect on HOMO-LUMO gaps, charge redistribution, structural modification and modification of vibrational modes has been extensively studied by varying the number and type of dopant. It turns out that the findings of this work give direction on how to design these clusters with the help of dopants for device applications. Further, endohedral Nitrogen clusters inside the fullerene molecule result in formation of polynitrogen complexes which are not possible in free space. This opens up another subject on an alternate source of green energy. Some new complexes have been proposed which can be prepared experimentally and hence encourages further inputs in this field.