Important roles of excitons in the linear and nonlinear optical properties of conjugated polymers and fullerene crystals are demonstrated on the basis of calculations using the conifguration interaction method for the Pariser-Parr-Pople model of interacting pi electrons. For polymers we study : (i) the excitonic features in third-order nonlinear optical susceptibilities; (ii) the optical properties of a relaxed exciton polaron; (iii) the role of biexciton-like states in the two-photon absorption spectrum. For fullerenes, we discuss the excitonic nature of photoexcited states in C-60 crystals.