Abstract: Diels-Alder reactions generally show large negative entropies of activation in line with the rigid transition state of the concerted mechanism. Thus, the experimental evidence is in line with a concerted n4s+n2s cycloaddition. However, the problems of orientation in the cycloaddition reaction have hardly seen studied. It is the intention of the present paper to put forward an explanation for the regioselectivity of reactions. A rule of orientation for the addition of unsymmetrical dienes and dienophiles, called here the 1-4; 2 - 1 rule is proposed, so that one can conveniently solve the problems of orientation of Diels-Alder reaction.