Suddenly, they’re everywhere: “Unicorns,” technology startups valued at more than $1 billion. How did these companies proliferate so quickly, and what does the flood of investments in these seemingly mythical businesses mean? And the obvious question: is it a sign of another bubble?

We asked the CEOs of these unicorns that very question. Most of them say no—which is unsurprising—but some say yes, and all of them cite different reasoning.