Recent research has highlighted the notion that people can make judgments and choices by means of two systems that are labeled here tacit (or intuitive) and deliberate (or analytic). Whereas most decisions typically involve both systems, this chapter examines the conditions under which each system is liable to be more effective. This aims to illuminate the age-old issue of whether and when people should trust intuition or analysis. To do this, a framework is presented to understand how the tacit and deliberate systems work in tandem. Distinctions are also made between the types of information typically used by both systems as well as the characteristics of environments that facilitate or hinder accurate learning by the tacit system. Next, several experiments that have contrasted intuitive and analytic modes on the same tasks are reviewed. Together, the theoretical framework and experimental evidence leads to specifying the trade-off that characterizes their relative effectiveness. Tacit system responses can be subject to biases. In making deliberate system responses, however, people might not be aware of the correct rule to deal with the task they are facing and/or make errors in executing it. Whether tacit or deliberate responses are more valid in particular circumstances requires assessing this trade-off. In this, the probability of making errors in deliberate thought is postulated to be a function of the analytical complexity of the task as perceived by the person. Thus the trade-off is one of bias (in implicit responses) versus analytical complexity (when tasks are handled in deliberate mode). Finally, it is noted that whereas much attention has been paid in the past to helping people make decisions in deliberate mode, efforts should also be directed toward improving ability to make decisions in tacit mode since the effectiveness of decisions clearly depends on both. This therefore represents an important frontier for research.