This article analyzes the leading theoretical approaches to Bayesian reasoning in the literature, and its main sources of empirical support. Many errors in probability estimates are attributed to people's inability to think in statistical tems when faced with information about a single event. Specifically, in situations where the normative model is Bayes's theorem, the well-known base rate neglect is analyzed both from rhe heuristic and biases approach, of Tversky and Kahneman, and from the frequentist hypothesis defended by Gigerenzer, Cosmides et al. The discrepancy between intuitions and formal mathenlatical reasoning is also analyzed through the studies with the three prisoners problem. Finally, we present the mental rnodel theoly of extensional probabilistic reasoning (Johnsorl-Laid et al., 1999) which explains how naive individuals can infer posterior probabilities without relying on Bayes 's theorern.