E d Hallen is an American technology entrepreneur and a co-founder of Klaviyo, a highly successful marketing automation and email platform. With a background in engineering, Hallen teamed up with his co-founder, Andrew Bialecki, to start the company in 2012. They identified a need for a more sophisticated and data-driven marketing platform specifically for e-commerce businesses.

Klaviyo's software allows online businesses to better understand their customers and send them personalized and automated marketing messages via email and SMS. The platform's ability to integrate deeply with e-commerce sites like Shopify made it an essential tool for thousands of online retailers. As a key leader in the company's product and engineering, Hallen helped build Klaviyo into a bootstrapped and highly profitable software-as-a-service (SaaS) company. The company later took on major funding at a multi-billion dollar valuation, turning its founders into billionaires.