No / There is considerable interest, both fundamental and technological, in understanding how additives and impurities influence crystal nucleation, and in the modulation of nucleation in a predictable way by using designer additives. An appropriate additive can promote, retard, or inhibit crystal nucleation and growth, assist in the selective crystallization of a particular enantiomer or polymorphic form, or enable crystals of a desired habit to be obtained.[1¿3] Applications involving additives include the control of the nucleation of proteins,[4] the inhibition of urinary-stone formation[5] and of ice formation in living tissues during cryoprotection,[6] their use as antifreeze agents in Antarctic fish,[7,8] the prevention of blockages in oil and gas pipelines as a result of wax precipitation[9] and gas-hydrate formation,[10] crystal-twin formation,[11] and as a possible basis for the antimalarial activity of some drugs.[12]We report herein the mode of action and explicit (apparently intuitive) rules for designing additive molecules for the modulation of crystal nucleation. The mode of action and the design features have been derived from molecular-dynamics simulations involving simple models.[13] These findings will help to rationalize how known nucleation inhibitors and modulators exert their effect and aid in the identification or design of new additives for the inhibition or promotion of nucleation in specific systems.