What is the mental representation of probability and how does it develop? What role do education and experience play in understanding probability? In this talk I will present the results of several experiments investigating probability judgments in children and adults. In the first part of the discussion I will present data from 3 Experiments in which adult participants perform a ratio comparison task. Findings revealed that people represent the probability of binary outcomes as proportions calculated over Approximate Number System representations and they sometimes use formally incorrect heuristic decision rules. In the second portion of the talk I will discuss the developmental trajectory of these computational abilities in children. Data from 6- to 12-year-olds suggests that by 7- to 8- years of age, children can accurately represent probabilities based on proportions and that the accuracy of probability judgments improves with age. Interestingly, this is around the same age that children are formally introduced to probability based on the Common Core State Standards. Finally, in the third section of the talk I will discuss the influence of presentation time and feedback on children’s use of heuristic decision rules. Preliminary findings from 2 completed experiments and one ongoing experiment reveals that children use heuristic rules to make binary probability judgments and that these rules can be overridden with an adequate amount and type of feedback. Together, these findings shed light on how the mind represents probability and how this representation is influenced by experience.