Toni Ko thought she was on top of her company’s finances. Then she woke up one morning in 2016 to find half a million pairs of extra sunglasses staring her in the face.

The founder and CEO of Los Angeles sunglasses company Perverse was by then an experienced–and successful–entrepreneur, having sold her previous company, NYX Cosmetics, to L’Oréal in 2014 for an estimated $400 million-plus. So Ko was not new to managing cash flow and inventory–and she thought she was pretty savvy about finances. Despite this, in Perverse’s early days Ko found herself making a very common mistake: buying more product than she could sell.

“I completely forgot my own advice,” admits Ko, 44. “With my first business, the inventory was a slow buildup over the years, so I never felt cash-strapped. But as I started my second business, I overpurchased.”

Ultimately, she had to trash more than 250,000 pairs of glasses. She wasn’t able to recoup the purchase price–but at least she saved tens of thousands of dollars in monthly storage fees. Ko swore that next time, she would remember her own advice. “As entrepreneurs, we know a lot and learn a lot–but sometimes we forget,” she says. “And this was a crucial one for me.”

As Ko has now learned twice over, money is at the root of every decision you make as a business owner. But saving and spending habits are often formed–or forgotten–long before you decide how much to pay your first employee. The salary you take for a new job will determine how much money you can set aside to start your first business. The looming sense of dread you feel when you can’t pay off a credit card bill at the end of the month could later remind you not to take on too much debt at your company.

To help guide you through the money trade­offs you face every day, Inc. asked founders, investors, and other business leaders to pass along the best piece of financial advice they’ve ever received. Some of these experts, like Goldie­Blox founder Debbie Sterling, are still building their first businesses; others, like Ko and cosmetics mogul Bobbi Brown, negotiated big-ticket sales of the companies they founded. And some, including Care.com founder Sheila Lirio Marcelo and Max Levchin of PayPal and now Affirm, have successfully navigated IPOs.

These entrepreneurs credit mentors, investors, the framers of the Constitution, and, like Ko, painfully lived personal experiences for their financial wisdom. But it’s often parental wisdom that ends up meaning the most. Just ask Shark Tank investor and Fubu founder Daymond John, whose mom took him to get his first credit card when he was 18–while warning him to never fall behind on payments because “the world is built on a credit system.”

Ben Chestnut, meanwhile, grew up watching his mother and sister run a beauty salon out of the family’s kitchen–and learned never to spend more than the cash he had on hand.

“My mother used to tell me, ‘You are the only person you can depend on to put food in your mouth,’ ” the 42-year-old co-founder and CEO of MailChimp recalls. “So in the early days of MailChimp, it never occurred to me to borrow money or get funding to grow my business.”

Sixteen years later, his Atlanta-based email marketing firm earned annual revenue of $403 million. “If I need to make more money,” Chestnut says, “I find a way to serve more customers–just like my mother taught me.”