The first part of this review presents the state of the art on two-photon absorbing molecules. Earlyworks concerned the optimization of dipolar push–pull molecules. Recently, the synthesis of linearand branched centrosymmetrical quadrupolar molecules led to higher nonlinearities, but still far from reachingthe fundamental limits of molecular two-photon cross-sections. The second part of this review summarizes our theoretical and experimental results on fluorene-basedoligomers, and branched oligomers (V-shape molecules and dendrimers). They are model molecules to investigatean alternative approach based on spatial assemblies of nonsubstituted π-electronsystems that are coupled by dipole–dipole interactions. In all geometries, two-photon absorptioncross-sections have superlinear dependencies that depend on coupling energies and oligomer sizes. Theseresults are well rationalized by an excitonic model based on N interactingthree-level systems. Analytical expressions for one-photon and two-photon energies and absorption strengthsare derived for linear oligomers. An accurate calculation of large exitonic systems is obtained by diagonalizingthe Hamiltonian operator on a reduced basis set.