Every tech startup wants to become a unicorn — a private company valued at more than $1 billion. Because they typically dominate a new business or technology, unicorns are able to raise billions of dollars from investors on the speculation that they will become the next Facebook or Google.

So called because of their rare nature, unicorns are actually becoming more common. As recently as five years ago, there were around 40 unicorns — non-public tech companies that were able to raise funds matching a valuation of at least $1 billion. Today, there are more than 140 unicorns. In fact, there are a number of companies that have yet to go public and that are valued at more than $10 billion. These so-called decacorns include companies such as Snapchat, Airbnb, and Uber, the largest decacorn in the world, valued of $66 billion. Based on estimates and data provided by private company research firm PrivCo, 24/7 Wall St. reviewed today’s 13 decacorns.

Generally, established companies are valued based on their current and future operations and their financials. Also, other companies in the same industry provide context for the valuation. However, in venture capitalism — and particularly for many of these unicorns — there are no existing examples to draw from because these companies are developing brand new technologies for which there is no precedent. The valuation is therefore often based on assumptions for the new industry and the company’s assumed capability to develop current and future operations.

Click here to see the world’s 13 most valuable startups.

For example, in the case of ride-sharing service Uber, the world’s highest valued startup, the company’s valuation is based on a fair amount of speculation because no company has ever competed with taxi services on a global scale before.

Due to the many unknowns and uncertainties, many argue that these unicorns are overvalued — some companies are valued at many times their funding totals — and that the recent explosion in the number of unicorns is unwarranted. Private spaceflight company SpaceX has raised $1.2 billion, but its most recent round of funding in October placed valued it at $15 billion. Chinese electronics company Xiaomi has raised an estimated $1.5 billion, a fraction of its valuation of $46 billion. Some speculate that the unicorn’s high valuation — five, 10, and even 30 times their capital — is the sign of another tech bubble.

24/7 Wall St. discussed unicorn valuations with Evan Danckwerth, senior financial analyst at PrivCo. Referring specifically to Uber’s $66 billion valuation Danckwerth said, “It’s not unheard of, but for a company where you don’t know where the revenue growth is exactly going to come from, it’s somewhat strange to see such a large valuation.” He added, “On the other hand, Uber has raised $16 billion, and that’s also unprecedented.”

While he acknowledged that some of the smaller unicorns may be overvalued, Danckwerth explained that there is increasing evidence that these numbers actually reflect the potential value of these new businesses.

Danckwerth explained that earlier this year, some major unicorns, including Snapchat, suffered from speculation of overvaluation. During that time, Snapchat’s valuation fell from $16 billion to around $13.7 billion. However, many of those companies have either returned to previous values or have since increased in value. In May, Snapchat raised funds based on a $20 billion value. This change, Dankwerth said, “means that there’s something there … [investors] are really looking at true revenue.”

Privco, a research company that specializes in data on non-public companies, provided 24/7 Wall St. with data on tech startups that have not yet gone public and have raised funds based on at least a $1 billion valuation in the most recent round of funding. The 13 companies on this list are the 13 companies valued at $10 billion or more. Privco also provided 24/ Wall st. with estimates of funds raised to date.