Molecularity: number of molecules participating in an elementary step. Always a positive integer (1 = unimolecular, 2 = bimolecular, 3 = trimolecular). Applies ONLY to elementary reactions, NOT to overall reactions. Molecularity > 3 is not observed — probability of >3 molecules colliding simultaneously is negligible. Order: exponent in the rate law. Can be 0, positive, negative, or fractional. Determined experimentally. Applies to overall reactions or elementary steps. For an elementary reaction: molecularity = order (always). For complex reactions: order is determined by the rate-determining step mechanism. Example: SN1 reaction is first order despite involving two reactants (rate-determining step is unimolecular).