High-level coupled-cluster methods have been used in conjunction with large basis sets to compute the lattice energy of crystalline benzene nearly within chemical accuracy (1 kcal/mol). Procedures for such computations for this and other organic crystals, and remaining sources of error, will be discussed. Ab initio quantum chemistry has also been used to elucidate the effect of substituents on intermolecular pi-pi interactions, and results appear to contradict the well-known Hunter-Sanders rules, which state that substituent effects can be explained solely on the basis of electrostatic terms. For the interaction of benzene with toluene, for example, dispersion effects dominate the difference in binding energy relative to the benzene dimer.