The fact that molecules now usually referred to as chiral were in the past mainly called optically active, is a reminder that the optical properties of chiral molecules were practically the only properties that were taken to be characteristic, and certainly the only ones that were at all easily measurable. Optical rotation, which is the longest known of the optical properties depends on the difference in refractive index for left and right-handed circularly polarised light. Once discovered, it proved an easy quantity to measure because it depends on the refractive index difference and does not have to be found by subtracting one very large quantity from another slightly different from it. Other optical measurements, such as optical rotatory dispersion (ORD), circular dichroism (CD) and even the second-order properties of induced circular dichroism (ICD) and magnetic circular dichroism (MCD) all have this same feature. The possibilities with isolated molecules probed by fields applied externally are limited. On the other hand, because a chiral molecule is the source of an external field which is inherently chiral, it must be coupled to a second chiral molecule with a strength that depends on the relative handedness of the pair. The field of a chiral molecule is here conceived in very general terms, including the virtual field associated with dispersive interactions and that for short range contacts as well as the more familiar electric field of permanent molecular charge distributions. The possibilities of investigation of chiral discrimination are at once much wider when such pairs or clusters are considered instead of isolated molecules. It is thus the condition that the probing field as well as the molecule probed should be chiral that gives a special importance to intermolecular interactions in such studies...