Koum was born in Kiev in the Ukraine, and when he was 16, he and his mother immigrated, leaving his father behind, in the hopes of finding a better life. They moved to Mountain View, Calif., where a government subsidy helped them get food stamps and an apartment. To help make ends meet, Koum worked as a janitor and his mother babysat.
The young man worked his way through college but ended up, like many famous entrepreneurs, quitting. He taught himself programming and eventually landed a job at Yahoo. When Koum became disenchanted, he quit and traveled the world in search of inspiration.
Then Apple's app store launched in 2009 and Koum saw a golden opportunity. He poured his time, effort and resources into building what would become WhatsApp.
In 2014, Koum and co-founder Brian Acton, another former Yahoo employee, sold WhatsApp to Facebook in a deal worth $19 billion. Today Koum is worth $9.3 billion.