This paper explains what "rainfall return period" means and how it's calculated, using the Philippines as a case study. Return periods (5-, 10-, 25-, 50-, and 100-year) are statistical estimates derived from decades of observed rainfall data, similar to how average typhoon frequency is calculated. Using data from PAGASA and the National Hydraulic Research Center, the paper illustrates how Tropical Storm Ondoy (2009) was classified as a 180-year rainfall event based on extrapolation from historical rainfall-duration curves at Science Garden Station, Quezon City. It cautions that return periods indicate statistical likelihood, not certainty — a "100-year rainfall" can recur far sooner, as demonstrated when the 2012 and 2013 Habagat monsoon rains rivaled Ondoy's intensity just years later, driven by anomalous weather patterns rather than typical storm tracks. The paper concludes that climate change may be shortening these return intervals, and emphasizes that disaster preparedness should not rely on statistical "safety" from recent extreme events.