Strong polymer fluorescence is linked to the nature of the lowest single excitation, S1. The interplay of electro-electron (e-e) correlations and alternation (delta) of transfer integrals t(1 +/- (delta) ) along the backbone leads to a dipole-forbidden S1 for small (delta) and a dipole-allowed S1 at large (delta) . We analyze the crossover from band to correlated behavior in alternating Hubbard and Pariser-Parr-Pople (PPP) chains in terms of elementary triplet and singlet excitations, make contact with exact results for regular ((delta) equals 0) and dimer ((delta) equals 1) chains, and show that molecular PPP parameters distinguish naturally between polymers like poly(p-phenylenevinylene), PPV, and polysilanes with a dipole-allowed S1 and others with dipole-forbidden S1. The phenyl rings in PPP provide a topological contribution to (delta) . The different energy thresholds of conjugated polymers for triplet, one and two-photon, and charge-carrying excitations arise from e-e correlations in half-filled bands. Correlations also rationalize the different relaxation energies of singlet, triplet, and charged bipolarons confined to trans stilbene or to anthracene. The coincidence of PPV photo- and electroluminescence implies that the relaxation energy of polarons is less than the Coulomb binding and relaxation energy of the single exciton S1.