Although considerable evidence suggests that forming an implementation intention increases rates of goal attainment, less research has examined the mechanisms that underlie these effects. The present research investigated the role of deliberative processes and accessibility of plan components as explanation for the relationship between implementation intentions and goal achievement. Study 1 used meta-analysis to quantify the effects of implementation intentions on goal intentions and self-efficacy. The results of 66 tests suggested that forming implementation intentions had negligible effects on both variables. Study 2 focused on the accessibility of plan components and found that the effect of implementation intentions on goal achievement was mediated simultaneously by the accessibility of specified situational cues and by the strength of the association forged between these cues and the intended response. These findings suggest that implementation intention effects are not explained by increased deliberation, but rather accrue from heightened accessibility of specified opportunities and strong opportunity-response links.