The Heartbleed vulnerability took the Internet by surprise in April of this year. The vulnerability was one of the most consequential in the history of the Internet, since it allowed attackers to potentially steal login credentials, cryptographic keys, and other private data from up to half of all popular HTTPS sites. In this talk, we take a detailed look at Heartbleed and its aftermath, based on comprehensive measurements and analysis that our research team performed over the past six months. We began tracking Heartbleed's impact within hours of its disclosure using massive ZMap scans and large network telescopes. This allowed us to track which sites remained vulnerable, observe certificate revocations, and monitor for large scale attacks in close to real time. Based on this data, we also conducted one of the largest ever mass vulnerability notifications, informing the network administrators for all devices still susceptible to Heartbleed across the entire IPv4 address space. Finally, we investigated the question of whether attackers knew about and exploited Heartbleed prior to its public disclosure---and we will present new details about this question in the talk. We hope that by learning from the Heartbleed security disaster, our community can prepare to respond more effectively to such events in the future.