Twitter Notifications Infrastructure enables hundreds of millions of users to stay informed about what’s going on in their Twitter world. Our systems process large volumes of data (aka the Twitter firehose) and deliver realtime and personalized notifications to all kinds of users, ranging from Katy Perry with ~95M followers to brand new users trying out our product for the very first time. We will give an overview of unique challenges in building our notifications infrastructure, present how we solved our challenges, and share key lessons learned along the way.