We outline a brief timeline of the history of ensemble prediction, which resulted in the setup of HEPEX, starting from the early research on atmospheric predictability and uncertainty in the 1960-70s, through the development of ensemble prediction methods for monthly to seasonal predictions in hydrology and meteorology in the US and UK in the 1980s, the development of the first medium-range (up to 10 days) operational weather ensemble prediction systems at ECMWF and NCEP in the 1990s, the first runs of EFAS with numerical weather ensemble predictions in 2005, up to the most recent developments on seamless prediction systems, ensemble short-range and nowcasting systems, coupled Earth systems and Global multi-model forecasting. The recognition of tradeoffs among different paradigms in hydrological forecasting is emphasized, and the transition from the first model-based, traditional "human-in-the-loop" flood forecasting approaches to more automated, "over-the-loop" probabilistic-based approaches and coupled NWP-hydrological modeling systems. Despite the initial focus on medium-range forecasting, HEPEX has been able to embrace a wide range of spatial and temporal scales. Today, model-based ensemble predictions are investigated for flash flood nowcasting and up to the long-range (several months) forecasting of water resources, and over spatial scales from small to large river systems controlled by different processes. Finally, it is important to note that HEPEX has always been an unfunded/volunteer effort - perhaps the longest in the history of hydrometeorological forecasting