Treadwell announced universal Windows apps which let people produce apps across phones, tablets and PCs. Developers will only have to design one app instead of doing individual apps for each device.
However, a developer can still customize their app to work and look different on each device, but without having to start from scratch.
The keynote is digging into the nitty gritty (geeky) details for developers, including a demo of Microsoft Visual Studio.
We're chatting about Microsoft office. Kirk Koenigsbauer, the vice president of Office, is on stage doing a demo of improved touch-first features on office.
Now Microsoft is spelling out how developers can take advantage of Cortana to create custom speech recognition commands. Cortana does the heavy lifting with natural language recognition. For a demo, they try to compose a tweet using Cortana. It gets the body right but botches the hashtag. #stillinbeta
"There's not better TV experience out there than XBox," says Terry Myerson. XBox is being used on 80 million TVs around the world and the average user is using it 5 hours a day.
Xbox is one of the platforms included in the Universal Apps development tool.
Talking about the Kinect: "It's a whole new way of interacting with the computer. You have the feeling that this is the future," says Myerson.
He's announcing the Kinect V2 for Windows. Improvements to skeletal tracking, facial tracking, gesture recognition and it works better in low light.
This keynote has been going on for two hours, and there's apparently another hour to go. I admire Microsoft's stamina -- no wimpy, 90-minute Apple keynotes for us! -- but come on: That's a long slog.
They are demonstrating a Big style giant piano on stage. "I think it's the first ever piano that runs Windows."
They are discussing developing for the Internet of things. "We are at a time where the devices we are programming to are going to change," says Myerson.
Live Tiles are coming to the Start menu. This will be available to all users with the Windows 8.1 update.
Windows will be available for free for phones and tablets smaller than 9-inches and for manufacturers working with the Internet of Things.
It's Build's Oprah moment: Everyone in the audience gets a free Xbox One. They also get a $500 gift card to the Microsoft store to buy a phone or tablet.
"We think at the screens get smaller, the devices get smaller, the cloud gets bigger."
Nokia has made over 80% of Windows Phones. Stephen Elop, the executive vice president of Nokia, is taking the stage to announce new Nokia and Lumia news.
Microsoft is still in the process of buying Nokia. The deal is nearly done.
All of Nokia's Lumia devices are getting the Windows Phone 8.1 update in a few months.
Elop is announcing new Nokia devices starting with the Nokia Lumia 930.
The 920 has a 20 megapixel camera and a new feature called Living Images.
It will be available for $599 around the world in June. That price is before taxes and carrier subsidies.
Two cheaper Lumia phones: The Nokia Lumia 630 and 635.
Some of the features Elop is touting: build-in fitness tracking features, dual SIM card support for people who use their devices for work and personal, Cortana, radio with playlists and the maps app.
Lumia 630 starts at $159 and is available in May.
New Microsoft CEO Nadella is finally taking the stage, a couple hours into this keynote.
He's taking a cue from Google's Larry Page and going straight into a Q&A session with developers. Only, these questions were picked and filmed ahead of time so there are no surprises.
Nadella lays out his argument for developing for Windows in response to a question from an Android developer. Windows has a "challenger mindset" and plans to innovate in multiple dimensions.
"What is Microsoft doing to compete against Google and Apple in the tablet space?"
The answer is not exactly shocking. Promises of great hardware and great software and tablets at every price point. Windows will continue to innovate with Surface, striving to make it the most productive tablet available. "The key thing we look at is what is the role of the tablet in a users' device family?"
The pre-recorded softball questions are setting Natella up perfectly to give his best the Windows sales pitch. Next: "What's the vision for Microsoft going forward?"
Now he's bantering with Cortana and asking it to cue up the exit music. It ended exactly on time, which is impressive.
And ... that's it. Thanks for following along! We'll have more Microsoft Build news, react and analysis on CNN.com/tech and CNNMoney.