Pamela Netzky and Andrew Friedman created Amplify Snack Brand in the hopes of finding a way to make a healthy popular snack. Their first product, called SkinnyPop Popcorn, used sunflower oil as a low-fat substitute for other types of cooking oil. SkinnyPop quickly built a loyal following, with sales of its product increasing by nearly $100 million from 2012 to 2014. In April 2015, Amplify acquired Paqui, a line of Non-GMO Project Verified tortilla chips, in a quest to build a hearty portfolio of "better for you" snacks. Amplify says it is always looking for healthier alternatives to classic nibbles, though its line of products in the salty-snack category remains small. Note: employee count as of Dec. 31, 2015

Cybersecurity firm Rapid7, founded by Alan Matthews, Tas Giakouminakis, and Chad Loder, touts its analytics-driven approach to protecting customer data as the key to its success. Rapid7's "threat exposure management" products assess a company's IT infrastructure to determine which parts are most vulnerable to an attack, and perform routine simulations to determine how well Rapid7's security measures would stand up against a hacker. But the cybersecurity industry is rapidly changing, and it remains to be seen how well Rapid7 can protect its customers against new kinds of phishing scams and hacking attempts. To keep growing, Rapid7 plans to increase the number of services available to customers. Note: employee count as of Dec. 31, 2015

This San Carlos, California-based biotech company makes non-invasive prenatal tests. Co-founder and CEO Matthew Rabinowitz saw his family devastated by the death of a baby with an untested genetic condition; he founded Natera to develop tests that can pick up on fetal chromosomal issues nine weeks into pregnancy. Natera's flagship test, known as an NIPT, analyzes traces of fetal cells that circulate in the mother's blood. Rabinowitz is tapping into a $665 million-plus global market of non-invasive prenatal testing, according to Persistence Market Research, and expanding: Natera hopes to unveil cancer diagnostics by the end of this year.

The fitness business is a popular one, and Rick Stollmeyer and Bob Murphy found a unique way to tap into the industry. MindBody makes software that helps fitness and wellness studios manage bill payments, organize schedules for instructors, and lets customers sign up for classes online. The company does have a history of losing money, as it has struggled with slow revenue growth in the past. But as MindBody sees more and more entrepreneurs getting into the health and wellness space, it remains confident in its business prospects. "We see huge growth opportunities--boutique fitness is still in the early stages of growth, and traditional doctors are embracing this idea of 'wellness,'" Stollmeyer told Inc. Note: employee count as of Dec. 31, 2015

Virtu Financial is a high-frequency trading firm founded by Vincent Viola and Douglas Cifu. Virtu programs algorithms that investment banks can use to determine when to buy and sell large quantities of stock at high speed, to ensure that there are always shares available to make a trade. But high-frequency trading got a bad rap in 2014, after the release of Michael Lewis's Flash Boys, which claimed that the practice "rigged the market," and launched an FBI investigation into its legality. It's unclear what high-frequency trading's reputation will be like in the future, but Virtu is trying to position itself as a "market maker" whose services help improve the health of global markets. Note: employee count as of Dec. 31, 2015