The PrESTO system is a peer review and peer assessment tool that allows large numbers of students to practice open-ended assignments, while keeping the workload of the teaching staff manageable. PrESTO was originally developed for a quantitative modeling course, but is applicable to any course with open-ended assignments that can be divided into successive steps. The software organizes the peer review and peer assessment workflow, and guides the students through the activities required in each step: review, give feedback on and assess the work of an anonymous predecessor, and then improve and extend it with a next step.