The diet comes from Beach Body, the same company that brought the world the notoriously intense at-homeworkouts Insanity and P90X. There's a nutrition plan that comes with the workouts, too.
21-Day Fix promises that in just 21 days, you'll see major results - a claim that has come under fire.
"21 Days is just what they call it - a fix. To maintain a healthy weight and mentality, you need to make a lifestyle change. I don't like the idea that the name deceives you a little bit here," Kait Hanson wrote on the Huffington Post.
But people post wild photos on Instagram to show how far they've come in a shockingly short period of time, suggesting that the plan could truly work.
Here's what you need to know about the diet trend.