A theoretical study of 3,4,9,10-Perylene Tetracarboxylic Diimide (PTCDI) has been carried out using density functional theory (DFT) and time-dependant DFT (TDDFT) methods. The HOMO→LUMO transition is found to have the lowest excitation energy, 505 nm with an oscillator strength f=0.6. The excited state geometry exhibits pronounced bond length alternation from the ground state. Franck-Condon factors are calculated using optimized ground and excited geometries and frequencies. Multiple mode vibrational coupling with the electronic transition is detected and assessed by Franck-Condon factors. The calculated absorption spectrum shows good agreement with the experimental data.