Today, Columbia Journalism Review published a lengthy piece about Substack, the newsletter publishing site that has recently become home to a number of journalists fleeing the constraints of larger media outlets.

Last month Glenn Greenwald announced he was leaving the Intercept, a site he co-founded, when he felt his editorial freedom was being challenged. In July, Andrew Sullivan was pushed out at New York Magazine because some portion of the staff wasn’t happy with his criticism of identity politics. And just last week, Matt Yglesias announced he was leaving Vox, again because it seems people were uncomfortable with some of his content. All three of them signed up with Substack, and Andrew Sullivan quickly became one of the most-subscribed producers on the site.