An opportunistic experiment is a type of randomized controlled trial that studies the effects of a planned intervention or policy change with minimal added disruption and cost. This guide defines opportunistic experiments and provides examples, discusses issues to consider when identifying potential opportunistic experiments, and outlines the critical steps to complete opportunistic experiments. It concludes with a discussion of the potentially low cost of conducting opportunistic experiments and the potentially high cost of not conducting them. Readers will also find a checklist of key questions to consider when conducting opportunistic experiments in the appendix.