On the basis of configuration interaction calculations, we first describe the nature of the lowest singlet and triplet excited states in oligothiophenes ranging in size from two to six rings. We calculate the vertical excitation energies from the singlet ground state S-0 to the first one-photon allowed singlet excited state S-1 as well as the energy difference between the ground state and the lowest triplet state T-1. The computed transition energies are in very good agreement with the measured values and indicate a strong confinement of the lowest triplet. We also uncover the nature of the higher-lying triplet excited state T-n that is coupled via a large oscillator strength to T-1. The evolution with chain length of the T-1-T-n excitation energies compares well with the experimental evolution based on photoinduced absorption data. Next, we investigate the geometry relaxation phenomena occurring in the S-1 and T-1 states; more pronounced and localized bond-length deformations are calculated in the triplet state than in the singlet, confirming the more localized character of T-1. We also analyze the influence on the lowest excited states of grafting electroactive end-groups on the conjugated path of terthiophene. Finally, the various mechanisms involved in the nonradiative decay of the singlet excitations are discussed, and results are presented as a guide toward the optimization of light emission efficiency in conjugated systems.