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They’re waiting in the wings, so to speak.

According to a PitchBook research report, there were 187 active unicorns (privately-held companies valued at $1 billion or more) in the U.S. by midyear in 2019, with an aggregate private valuation of just over $600 billion (just slightly down from the previous peak in 2018 of $603.3 billion).

“This is really a new phenomenon,” Jane Leung, the managing director and chief investment officer at Scenic Advisement, told Fortune. “You really didn’t have companies this size coming to market five years ago even.”

The glut of unicorns, according to Leung, can be attributed to the huge inflows of capital into the private markets since 2008, primarily from institutions. “The private markets are so big because the money is really flowing there,” Leung says. “This really stems back to the [global financial crisis] in 2008—this search for yield after the crisis.”

In fact, Scenic’s Leung suggests that massive capital inflows (in the hundreds of billions) into private markets in recent years are allowing many companies that would ordinarily feel pressured to IPO stay private longer. But, as is evidenced by companies like WeWork, she says, “even if [these companies] never go public, they need to have discipline.”

We looked at five of the most valuable unicorns—and (where applicable) their IPO plans.

1—The We Company ($47 billion)

It’s probably the most scrutinized and debated company trying to IPO this year, but once upon a time, WeWork’s last funding round (completed by SoftBank in January this year for $5 billion, according to PitchBook data) pushed We’s valuation up to a whopping $47 billion.

The company received the majority of its funding from the Japanese conglomerate’s Vision Fund, which is now getting cold feet over the looming valuation discount—although CNBC reported that due to some fine print, a flop IPO could actually trigger $400 million worth of additional shares to SoftBank through a so-called “ratchet” provision.

Still, despite raising around $12 billion from private investors and achieving a sky-high premium valuation, the co-working company has been struggling to drum up sufficient investor interest in the company’s prospective stock—and soothe fears over a variety of issues plaguing the company including cash burn, corporate governance, and liabilities. In fact, the company officially announced Monday that We Co. will be withdrawing their S-1 filing from the Securities and Exchange Commission, “to focus on our core business,” WeWork’s new co-CEOs said in a statement.

So at least for now, a WeWork IPO is not in the works.

2—JUUL Labs ($38 billion)

The e-cigarette-maker may be in hot water with the possibility of an FDA ban on their USB-like vape (as well as a federal criminal probe), but JUUL Labs Inc. is still valued at a hefty $38 billion by private investors.

The vape company raised a total of $14.15 billion during seven funding rounds, the latest of which (a venture capital round that raised $785.2 million in August from Poseidon Asset Management and Proioxis Ventures) earned the e-cigarette maker its high valuation, according to data from PitchBook. The company’s most recent transaction was a secondary transaction where Valor Equity Partners sold a stake in the company to an undisclosed buyer.

JUUL was spun-off from PAX Labs in 2017. But as vaping-related illnesses and even deaths continue to increase, some are questioning the unicorn’s valuation. In fact, one of JUUL’s principle investors, tobacco behemoth Altria, invested some $12.8 billion (a 35% stake) in the company in December—but has since seen its own market cap sharply decline by around 18% year-to-date amid JUUL’s troubles.

3—Stripe ($35.3 billion)

Raising a total of $1.29 billion, Stripe is valued at $35.25 billion—making the company’s founders John and Patrick Collison the richest self-made billionaires in Ireland.

The payments giant is seemingly exciting private investors, announcing some $250 million raised in their latest funding round last Thursday in a deal led by Andreessen Horowitz, General Catalyst, and Sequoia Capital, according to PitchBook.

The payments processing company also recently announced it launched Stripe Capital, a loan-making unit.

4—SpaceX ($33.4 billion)

Until recently, Musk’s brainchild SpaceX held the number three spot among the highest-valued unicorns in the private markets. But even at number four, the space company still has a sky (or space)-high valuation—some $33.4 billion.

SpaceX raised an additional $314.15 million in the company’s 13th funding round in June, according to PitchBook, putting Musk’s company at its current valuation.

However, Musk (and SpaceX) have recently come under fire over allegations that the CEO used the space company and Tesla to bail out Musk’s cousin’s company, SolarCity, in 2016 for $2.6 billion—although the CEO says he recused himself from the final decision to acquire SolarCity.

5—Airbnb ($31 billion)

The global lodging company recently announced their plans to file for an IPO in 2020. And according to Airbnb’s latest funding round, the company is worth some $31 billion.

Airbnb completed a Series F funding round (the company’s 7th) in 2017 to the tune of $1 billion led by CapitalG and Technology Crossover Ventures among others—pushing the lodging company’s valuation up to $31 billion, according to PitchBook. The company has raised $4.40 billion to date.

The home sharing service would be among the recent tech companies to IPO, but some on the Street seem to think it will follow a better path than those like Uber and Lyft in the public markets. In fact, in the 2nd quarter, the company said it had “substantially more than $1 billion” in revenue from over 7 million listings in over 100,000 cities worldwide—and even claimed it was profitable (excluding interest, taxes, and other costs).