The authors review the Implicit Association Test (IAT), its use in marketing, and the methodology and validity issues surrounding it. They focus then on a validity problem that has not been investigated so far, the impact of cognitive inertia on IAT-effects. Cognitive inertia denotes the difficulty in switching from one categorization rule to the opposite categorization rule. This difficulty causes IAT-effects to depend on the order in which the two IAT-blocks are administered. In study 1, an IAT-effect is observed when the ‘compatible’ block precedes the ‘incompatible’ block, but not when the ‘compatible’ block follows the ‘incompatible’ block. In studies 2 and 3, the IAT-effect changes its sign when the order of the blocks is reversed. Cognitive inertia distorts individual IAT-scores and diminishes correlations between IAT-scores and predictor variables when block-order is counterbalanced between-subjects. Study 4 shows that counterbalancing block-order repeatedly within-subjects can eliminate cognitive inertia effects on the individual level. The authors conclude that researchers should either interpret IAT-scores on the aggregate level or, when individual IAT-scores are of interest, counterbalance block-order repeatedly within-subjects.