When Bill Magnuson snagged an internship working for Google's Android division in 2009, the tech giant's smartphone was only six months old. But as mobile usage continued to grow, Magnuson realized that apps would eventually need to use a more sustainable monetization model to survive. Two years later, along with co-founders Jon Hyman and Mark Ghermezian, he built a mobile marketing platform called Appboy. The software helps companies learn more about phone users who download their app. Appboy collects the data by building out consumer profiles, which brands then use to send push notifications targeting specific groups. It took two years to get customer traction. Clients were skeptical that it was worth trying to convince customers to do anything besides spend 99 cents to download an app, says Magnuson. But persistence paid off for Appboy, whose client list now includes Jet.com, Delivery Hero, and SoundCloud. The average contract size for an Appboy customer is six figures. So far, the company has raised $42.5 million in total from venture capitalists. --Anna Hensel

Laura Behrens Wu started a business to solve a problem--a problem for the business she was then running. In 2013, Wu--a German graduate student and tech worker living in San Francisco--launched an online store, selling sustainable handbags. She used Shopify to process sales and Stripe for payments. Problem was, frequent trips to the post office became a time suck. "We were making two to three trips a day," says Wu. She figured she wasn't the only one feeling the pain--that same year, she abandoned her shop to found Shippo. Before Shippo, she says, it was next to impossible to easily compare prices, routes, and times among carriers like UPS and FedEx. That's just not tenable, Wu thought. "In e-commerce, shipping is such a cost factor," she says. "So we started to look into the inefficiencies of shipping." Today, some 10,000 customers use the company's platform. --Graham Rapier

Growing up in a small Missouri town without any sisters, Casey Lewis felt alienated from other women. "Teen magazines were my gateway into women's health and the world," she says. Her experience became the launch point for Clover Letter, an email newsletter and community for teenage girls. Her readers, Lewis explains, probably don't visit traditional outlets like The New York Times or CNN, but they have a desire to stay informed on the most pressing current events. In 2015, Lewis and her co-founder, Liza Darwin, quit their media jobs to start Clover Letter, which now counts more than 50,000 readers. Based in New York City, Clover Letter generates revenue mostly by charging its clients, which include Penguin and Dove, a fee to run branded content. Still, one major challenge has been convincing investors that Generation Z does, in fact, spend time on email--as opposed to Snapchat, which now has more than 150 million daily active users. "Every investor we've met with said we needed to get on Snapchat Discover," says Lewis. "But we said we're going to try this other thing." Clover Letter recently closed a seed funding round of $300,000. The founders, both 29, say the U.S. presidential election--as well as the new administration--has given them a renewed sense of purpose for their readers. "As we're watching the government implode," says Darwin, "these girls still find optimism." --Zoë Henry

In Uganda alone, nearly 600,000 newborns require medical intervention for complications at birth. That's a problem that Teresa Cauvel and Sona Shah are on a mission to solve. Founded in 2015, their health tech startup, Neopenda, makes wearables that monitor four newborn vitals: heart rate, respiration, blood oxygen saturation, and temperature. Each battery-charged sensor is integrated into a baby hat and wirelessly transmits data to a central monitor. "A lot of my inspiration comes from the nurses and clinicians who tirelessly care for hundreds of newborns every week," says Shah. Given that many hospitals in the developing world are understaffed, Neopenda offers the solution of time and cost-efficiency. A typical--and more advanced--monitor in the U.S. costs $2,500, but Shah and Cauvel aim to reduce Neopenda's price to $75 per unit. The team plans to begin test trials in Uganda this year. Should Neopenda receive approval from the Uganda government, it can expand across East Africa. "The progress we've made and support we've received in less than two years has been exciting," says Cauvel, who notes that Neopenda's board consists of a diverse crowd of experts, from a clinical professor of pediatrics at U.C. San Diego to the SVP of external affairs at the TB Alliance. "It's our sole mission to give millions of newborns the healthy start to life they deserve." --Vanna Le

Every college student knows that the first day moving into the dorm is exciting, but kind of a pain in the neck. The sheets don't fit. The concept of a shower caddy is completely new. And suddenly, a laundry hamper seems like a necessity. Unlike most college students, cousins Shanil Wazirali and Sagar Hemani think they've got a fix. Their company, Roomify, consolidates student shopping needs in one big box of stuff, all color-coordinated, all delivered to their dorm. Its best-selling box is the $299 "Doin' It Big," which includes 42 items ranging from bedding to a bulletin board. The five-person company designs and manufactures everything itself, which allows it to keep prices low. Starting out, the founders had a leg up: Their fathers ran a family business that sourced white-label hardware and school supplies. "They guided us and told us which factories to work with," says Wazirali. "We were able to find out exactly where companies like Target were manufacturing." In March, Roomify closed on $1.1 million in financing. "Every year we sell out," says Wazirali. "And every year we're like, man, we're on to something." --Kimberly Weisul