While investors will focus on Amazon's giant e-commerce business when it reports earnings Thursday afternoon, analysts say its future is tied to Prime.
For $99 per year, Prime members get free two-day shipping, free and unlimited streaming of movies and TV shows and free access to music playlists. It's growing into a big business.
Analysts at Deutsche Bank estimate there are now 32 million Prime members, or about 13 percent of Amazon's total customer base. And these customers are the company's most loyal and frequent shoppers.
Prime customers spend around $2,500 per year on Amazon, according to analysts. That's about eight times the spending of nonprime customers.
And Deutsche Bank estimates that Prime could reach 100 million members by 2020. If that happens, Amazon would have one of the largest membership clubs in retail or any industry.
But not every Prime member is thrilled with the experience. In a recent survey by UBS, 42 percent of Prime members were "very satisfied," but 58 percent had some doubt about the service.
That's probably not surprising, given that Amazon recently raised the membership price from $79 to $99. With that hike, customers expect many more services.
So how is Amazon going to entice more customers to join while keeping existing customers happier?
Analysts list a few options. The company could increase the number of items available for free shipping, increase streaming content or provide even faster shipping times.
But that all costs a lot of money, which would pressure margins in ways that Wall Street isn't currently expecting, analysts said.
Already, that tension between growth and margins is frustrating investors, who have sent Amazon's stock down some 10 percent this year.
If Prime is at the center of Amazon's growth strategy, then content and shipping terms will need to improve, analysts said. It will have to provide a more compelling value proposition over its competitors, who now include everyone from Netflix to Wal-Mart.
As for the company's earnings, which will be revealed after the closing bell, analysts polled by Thomson Reuters expect a loss of 15 cents per share on revenue of $19.34 billion. That would be 23 percent higher than the $15.70 billion in last year's second quarter. The company had a loss of 2 cents per share in that quarter a year ago.
—By CNBC's Mark Berniker and Josh Lipton