Natural language processing ranks as the fastest-growing, in-demand skill in the US online jobs market as products utilizing intelligent, voice-activated assistants like Amazon Echo and Google Home see increased adoption, according to the fourth-quarter skills index released today by Upwork. The list includes the top 20 hottest skills on its online staffing platform. Swift, which is used to develop the Apple Watch, took the No. 2 spot on the list.

Data science was the fastest-growing category for the second consecutive quarter.

The 20 hottest skills, all of which experienced more than 100% year-over-year growth:

1. Natural language processing
2. Swift
3. Tableau
4. Amazon Marketplace Web Services
5. Stripe
6. Instagram marketing
7. MySQL programming
8. Unbounce
9. Social media management
10. AngularJS
11. Business consulting
12. Machine learning
13. Brand strategy
14. 3D rendering
15. Information security
16. R development
17. Node.js development
18. User experience design
19. Bluetooth
20. Zendesk customer support

“Freelancers represent the future of not only the US workforce but also of many of the innovations we’re seeing globally,” said Upwork CEO Stephane Kasriel. “The skills on this list are a snapshot of new and emerging technologies that will shape our world and create more job opportunities. As society becomes more and more specialized, businesses will rely on freelancers with the most current skills. Take natural language processing, for example — hiring freelancers allows businesses access to sharply-honed skills that simply aren’t available elsewhere.”

The Upwork Skills Index calculates growth rates based on freelancer billings through Upwork in the fourth quarter 2016 versus the same quarter of the prior year.