Conjugated polymers have been intensively studied in connection with high conductivity on doping, large nonlinear optical responses in their semiconducting state, and recently as novel light-emitting diodes (LEDs). Many different polymers are realized in Fig. 1 on varying the substituents R, including the best characterized examples[2,3] that have centrosymmetric conjugated backbones in their idealized extended geometry. The PS and PPV families fluoresce strongly, the PT family Moderately, and the PDA's or PA hardly at all. We have related [4,5] polymer fluorescence to the nature of the lowest singlet excited state, Si, which in C2h symmetry is either a dipole-allowed Bu or a two-photon allowed Ag state. Since the ground state is Ag, S1 is either 1B or 2A and fluorescence requires E (1B) < E(2A). The idea is simply that radiationless decay wins out for the low excitation energies and many vibrational degrees of freedom of conjugated polymers unless Si has a strong transition dipole to the ground state.