The 2021 unprecedented Pacific Northwest heatwave broke temperature records by extraordinary amounts. Impacts included hundreds of deaths, mass-mortalities of marine life, increased wildfires, reduced crop and fruit yields, and river flooding. In late June 2021 a heatwave of unprecedented magnitude impacted the Pacific Northwest region of Canada and the United States. Many locations broke all-time maximum temperature records by more than 5 & DEG;C, and the Canadian national temperature record was broken by 4.6 & DEG;C, with a new record temperature of 49.6 & DEG;C. Here, we provide a comprehensive summary of this event and its impacts. Upstream diabatic heating played a key role in the magnitude of this anomaly. Weather forecasts provided advanced notice of the event, while sub-seasonal forecasts showed an increased likelihood of a heat extreme with lead times of 10-20 days. The impacts of this event were catastrophic, including hundreds of attributable deaths across the Pacific Northwest, mass-mortalities of marine life, reduced crop and fruit yields, river flooding from rapid snow and glacier melt, and a substantial increase in wildfires-the latter contributing to landslides in the months following. These impacts provide examples we can learn from and a vivid depiction of how climate change can be so devastating.