This Guideline has been designed for professionals with a role in the planning, design, management, inspection, and regulation of stormwater, drainage, wastewater, and flood management systems. It is not a design text book, but rather a resource for understanding the derivation, and application in water system planning and design, of rainfall intensity-duration-frequency (IDF) information. IDF information describes the frequency (probability of occurrence) of extreme rainfall events of various rates and durations. This Guideline is intended to equip the reader with the ability to ensure that rainfall IDF characteristics are properly considered in the planning and design of water infrastructure. In particular, this can be provided through familiarization with the assumptions contained within current IDF information, and clarification regarding any limitations of this information. The effects of climate change on the frequency and intensity of extreme rainfall events are also addressed.